QC 523 



1895a 
(Copy 1 



A * -. 

or 




A^ ** 1 



O •/ , 






0c> 






- 









^ ^ 






_ 



























o v 




> 
























*P 







































, 



NEW AND REVISED EDITION. 

ELEMENTARY LESSONS 

IN 

ELECTRICITY AND MAGNETISM, 

By SILVANUS P. THOMPSON, D.Sc, F.R.A.S., 

needs no introduction to teachers of these subjects. Its 
explanations are extremely clear and models of terseness, 
contain sufficient reference to modern appliances to make 
them valuable to those who wish to get a knowledge of 
practical electrical work, and are written throughout in a 
sound scientific spirit. To further extend its already wide 
usefulness there is in preparation, to be published the first 
of January, 

a new, revised, and enlarged edition, 

in which every paragraph has been gone over with a view 
to bringing the book abreast not merely of modern discov- 
eries and appliances, but particularly of such research and 
invention as is of special value to American students. 

This edition is largely rewritten, contains upwards of a 
hundred pages of entirely new matter, and will be wholly 
reset in this country. Owing to its increased size it will be 
necessary to advance the price slightly, but it is expected 
that the retail cost will not exceed $1.40. 

For introductory rates, specimen copies, etc., apply to 
the publishers, 

MACMILLAN & CO., 

66 FIFTH AVENUE, NEW YORK. 



ELEMENTARY LESSONS 



ELECTRICITY & MAGNETISM 



BY 

SILVANUS P. THOMPSON, 

D.Sc, B.A., F.E.8., F.B.A.S. 

PRINCIPAL OF AND PROFESSOR OF PHYSICS IN THE CITY AND GUILDS OF 

LONDON TECHNICAL COLLEGE, FINSBURY ; 

LATE PROFESSOR OF EXPERIMENTAL PHYSICS EN 

UNIVERSITY COLLEGE, BRISTOL 



NEW EDITION, REVISED THROUGHOUT 
WITH ADDITIONS 



Ncfa gotfe 
MACMILLAN AND CO. J - ^ V 



AND LONDON 

1895 

All rights reserved 






\ 



Copyright, 1894, 
By MACM1LLAN AND CO. 



Norbjooti Press : 

J. S. Cushing & Co. — Berwick & Smith. 

Boston, Mass., U.S.A. 



ELEMENTAEY LESSONS 

ON 

ELECTRICITY & MAGNETISM 
$art jfirst 

CHAPTER I 

FRICTIOXAL ELECTRICITY 

Lesson I. — Electric Attraction and Repulsion 

1. Electricity. — Electricity is the name given to an 
invisible agent known to us only by the effects which it 
produces and by various manifestations called electrical. 
These manifestations, at first obscure and even mysterious, 
are now well understood; though little is yet known of 
the precise nature of electricity itself. It is neither 
matter nor energy ; yet it apparently can be associated 
or combined w T ith matter; and energy can be spent in 
moving it. Indeed its great importance to mankind 
arises from the circumstance that by its means energy 
spent in generating electric forces in one part of a system 
can be made to reappear as electric heat or light or work 
at some other part of the system ; such transfer of energy 
taking place even to very great distances at an enor- 
mous speed. Electricity is apparently as indestructible as 

B 1 



2 ELECTRICITY AND MAGNETISM part i 

matter or as energy. It can neither be created nor 
destroyed, but it can be transformed in its relations to 
matter and to energy, and it can be moved from one place 
to another. In many ways its behaviour resembles that 
of an incompressible liquid ; in other ways that of a 
highly attenuated and weightless gas. It appears to exist 
distributed nearly uniformly throughout all space. Many 
persons (including the author) are disposed to consider it 
as identical with the luminiferous ether. If it be not the 
same thing, there is an intimate relation between the two. 
That this must be so, is a necessary result of the great 
discovery of Maxwell — the greatest scientific discovery of 
the nineteenth century — that light itself is an electric 
phenomenon, and that the light-waves are merely electric, 
or, as he put it, electromagnetic waves. 

The name electricity is also given to that branch of 
science which deals with electric phenomena and theories. 
The phenomena, and the science which deals w r ith them, 
fall under four heads. The manifestations of electricity 
when standing still are different from those of electricity 
moving or flowing along : hence we have to consider 
separately the properties of (i.) statical charges, and those 
of (ii.) currents. Further, electricity whirling round or in 
circulation possesses properties which were independently 
discovered under the name of (iii.) magnetism. Lastly, 
electricity w T hen in a state of rapid vibration manifests 
new properties not possessed in any of the previous states, 
and causes the propagation of (iv.) leaves. These four 
branches of the science of electricity are, however, closely 
connected. The object of the present work is to give the 
reader a general view of the main facts and their simple 
relations to one another. 

In these first lessons we begin with charges of 
electricity, their production by friction, by influence, and 
by various other means, and shall study them mainly by 
the manifestations of attraction and repulsion to which 
they give rise. After that we go on to magnetism and 



CHAP. I 



ELECTRIC ATTRACTION 



currents, and the relations between them. The subject of 
electric waves is briefly discussed at the end of the book. 
2. Electric Attraction. — If you take a piece of seal- 
ing- wax, or of resin, or a glass rod, and rub it upon a 
piece of flannel or silk, it will be found to have acquired 
a property which it did not previously possess : namely, 
the power of attracting to itself such li^ht bodies as chaff, 
or dust, or bits of paper (Fig. 1). This curious power 




Fig. l. 

was originally discovered to be a property of amber, or, 
as the Greeks called it, yjXtKTpov, which is mentioned by 
Thales of Miletus (b.c. 600), and by Theophrastus in his 
treatise on Gems, as attracting light bodies when rubbed. 
Although an enormous number of substances possess this 
property, amber and jet w^ere the only two in which its 
existence had been recognized by the ancients, or even 
down to so late a date as the time of Queen Elizabeth. 



ELECTRICITY AND MAGNETISM part i 




About the year 1600, Dr. Gilbert of Colchester discovered 

by experiment that not 
only amber and jet, but a 
very large number of sub- 
stances, such as diamond, 
sapphire, rock-crystal, glass, 
sulphur, sealing-wax, resin, 
etc., which he styled elec- 
trics* possess the same pro- 
perty. Ever since his time 
the name electricity f has 
been employed to denote the 
agency at work in producing 
these phenomena. Gilbert 
also remarked that these ex- 
periments are spoiled by the 
*' ' presence of moisture. 

3. Further Experiments. — A better way of observ- 
ing the attracting force is to employ a small ball of elder 
pith, or of cork, hung 
by a fine thread from a 
support, as shown in 
Fig. 2. A dry warm 
glass tube, excited by 
rubbing it briskly with 
a silk handkerchief, 
will attract the pith- 
ball strongly, showing 
that it is highly electri- 
fied. The most suit- 
able rubber, if a stick 
of sealing-wax is used, 
will be found to be 
flannel, woollen cloth, or, best of all, fur. Boyle discovered 

* " Electrica ; quae attrahunt eadem ratione ut electrum " (Gilbert), 
t The first work in which this terra was used is that of Eobert Boyle, 
07i the Mechanical Production of Electricity, published at Oxford in 1675. 




Fig. 3. 



chap, i ELECTRIFICATION BY FRICTION 5 

that an electrified body is itself attracted by one that 
has not been electrified. This may be verified (see 
Fig. 3) by rubbing a stick of sealing-wax, or a glass 
rod, and hanging it in a wire loop at the end of a silk 
thread. If, then, the hand be held out towards the 
suspended electrified body, the latter will turn round 
and approach the hand. So, again, a piece of silk ribbon, 
if rubbed with warm indiarubber, or even if drawn 
between two pieces of warm flannel, and then hung up 
by one end, will be found to be attracted by objects 
presented to it. If held near the wall of the room it will 
fly to it and stick to it. With proper precautions it can 
be shown that both the rubber and the thing rubbed are 
in an electrified state, for both will attract light bodies ; 
but to show this, care must be taken not to handle the 
rubber too much. Thus, if it is desired to show that 
when a piece of fur is rubbed upon sealing-wax, the fur 
becomes also electrified, it is better not to take the fur in 
the hand, but to cement it to the end of a glass rod as a 
handle. The reason of this precaution will be explained 
toward the close of this lesson, and more fully in Lesson 
IV. 

A large number of substances, including iron, gold, 
brass, and all the metals, when held in the hand and 
rubbed, exhibit no sign of electrification, — that is to say, 
do not attract light bodies as rubbed amber and rubbed 
glass do. Gilbert mentions also pearls, marble, agate, 
and the lodestone, as substances not excited electrically 
by rubbing them. Such bodies were, on that account, 
formerly termed non-electrics ; but the term is erroneous, 
for if they are mounted on glass handles and then rubbed 
with silk or fur, they behave as electrics. 

4. Electric Repulsion. — When experimenting, as in 
Fig. 1, with a rubbed glass rod and bits of chopped paper, 
or straw, or bran, it will be noticed that these little 
bits are first attracted and fly up towards the excited rod, 
but that, having touched it, they are speedily repelled 



6 



ELECTRICITY AND MAGNETISM part i 




and fly back to the table. To show this repulsion better, 
let a small piece of feather or down be hung by a silk 

thread to a support, and 
let an electrified glass rod 
be held near it. It will 
dart towards the rod and 
stick to it, and a moment 
later will dart away from 
it, repelled by an invisible 
force (Fig. 4), nor will it 
again dart towards the 
rod. If the experiment 
be repeated with another 
feather and a stick of 
sealing-wax rubbed on 
flannel the same effects 
will occur. But, if now 
the hand be held towards 
the feather, it will rush 
toward the hand, as the rubbed body (in Fig. 3) did. 
This proves that the feather, though it has not itself been 
rubbed, possesses the 
property originally 
imparted to the rod 
by rubbing it. In 
fact, it has become 
electrified, by having 
touched an electrified 
body which has given 
part of its electricity 
to it. It would ap- 
pear then that two 
bodies electrified with 
the same electrifica- 
tion repel one an- 
other. This may be confirmed by a further experiment. 
A rubbed glass rod, hung up as in Fig 3, is repelled by a 



Fig. 4. 





Fig. 5. 



chap, i OPPOSITE ELECTRIC STATES 7 

similar rubbed glass rod ; while a rubbed stick of sealing- 
wax is repelled by a second rubbed stick of sealing-wax. 
Another way of showing the repulsion between two 
similarly electrified bodies is to hang a couple of small 
pith-balls, by thin linen threads to a glass support, as 
in Fig. 5, and then touch them both with a rubbed glass 
rod. They repel one another and fly apart, instead of 
hanging down side by side, while the near presence of 
the glass rod will make them open out still wider, for 
now it repels them both. The self-repulsion of the parts 
of an electrified body is beautifully illustrated by the 
experiment of electrifying a soap-bubble, which expands 
when electrified. 

5. Two Kinds of Electrification. — Electrified bodies 
do not, however, always repel one another. The feather 
which (see Fig. 4) has been touched by a rubbed glass 
rod, and which in consequence is repelled from the 
rubbed glass, will be attracted if a stick of rubbed seal- 
ing-wax be presented to it ; and conversely, if the feather 
has been first electrified by touching it with the rubbed 
sealing-wax, it will be attracted to a rubbed glass rod, 
though repelled by the rubbed wax. So, again, a rubbed 
glass rod suspended as in Fig. 3 will be attracted by a 
rubbed piece of sealing-wax, or resin, or amber, though 
repelled by a rubbed piece of glass. The two pith-balls 
touched (as in Fig. 5) with a rubbed glass rod fly from 
one another by repulsion, and, as we have seen, fly wider 
asunder when the excited glass rod is held near them ; 
yet they fall nearer together when a rubbed piece of 
sealing-wax is held under them, being attracted by it. 
Symmer first observed such phenomena as these, and 
they were independently discovered by Du Fay, who 
suggested in explanation of them that there were two 
different kinds of electricity which attracted one another 
while each repelled itself. The electricity produced on 
glass by rubbing it with silk he called vitreous electricity, 
supposing, though erroneously, that glass could yield no 



8 ELECTRICITY AND MAGNETISM part i 

other kind ; and the electricity excited in such substances 
as sealing-wax, resin, shellac, indiarubber, and amber, 
by rubbing them on wool or flannel, he termed resinous 
electricity. The kind of electricity produced is, however, 
found to depend not only on the thing rubbed but on the 
rubber also; for glass yields "resinous" electricity when 
rubbed with a cat's skin, and resin yields "vitreous" 
electricity if rubbed with a soft amalgam of tin and 
mercury spread on leather. Hence these names have 
been abandoned in favour of the more appropriate terms 
introduced by Franklin, who called the electricity excited 
upon glass by rubbing it with silk positive electricity, and 
that produced on resinous bodies by friction with wool or 
fur, negative electricity. The observations of Symnier 
and Du Fay may therefore be stated as follows: — Two 
positively electrified bodies apparently repel one another: 
two negatively electrified bodies apparently repel one 
another : but a positively electrified body and a negatively 
electrified body apparently attract one another. It is 
now known that these effects which appear like a repul- 
sion and an attraction between bodies at a distance from 
one another are really due to actions going on in the 
medium between them. The positive charge does not 
really attract the negative charge that is near it ; but 
both are urged toward one another by stresses in the 
medium in the intervening space. 

6. Simultaneous Production of both Electrical States. 
— Neither kind of electrification is produced alone ; 
there is always an equal quantity of both kinds pro- 
duced ; one kind appearing on the thing rubbed and an 
equal amount of the other kind on the rubber. The 
clearest proof that these amounts are equal can be given 
in some cases. For it is found that if both the — electricity 
of the rubber and the 4- electricity of the thing rubbed be 
imparted to a third body, that third body will show no 
electrification at all, the two equal and opposite electrifica- 
tions having exactly neutralized each other. A simple 



chap, i THEORIES OF ELECTRICITY 9 

experiment consists in rubbing together a disk of sealing- 
wax and one covered with flannel, both being held by 
insulating handles. To test them is required an insulated 
pot and an electroscope, as in Fig. 29. If either disk be 
inserted in the pot the leaves of the electroscope will 
diverge ; but if both are inserted at the same time the 
leaves do not diverge, showing that the two charges on 
the disks are equal and of opposite sign. 

In the following list the bodies are arranged in such an 
order that if any two be rubbed together the one which 
stands earlier in the series becomes positively electrified, 
and the one that stands later negatively electrified : — 
Fur, wool, ivory \ glass, silk, metals, sulphur, indiarubber, 
guttapercha, collodion, or celluloid. 

7. Theories of Electricity. — Several theories have 
been advanced to account for these phenomena, but all 
are more or less unsatisfactory. Symmer proposed a 
" two-fluid" theory, according to which there are two 
imponderable electric fluids of opposite kinds, which 
neutralize one another when they combine, and which 
exist combined in equal quantities in all bodies until 
their condition is disturbed by friction. A modification 
of this theory was made by Franklin, who proposed 
instead a " one-fluid " theory, according to w r hich there 
is a single electric fluid distributed usually uniformly 
in all bodies, but which, when they are subjected to 
friction, distributes itself unequally between the rubber 
and the thing rubbed, one having more of the fluid, the 
other less, than the average. Hence the terms positive 
and negative, which are still retained; that body wmich is 
supposed to have an excess being said to be charged with 
positive electricity (usually denoted by the plus sign +), 
while that which is supposed to have less is said to be 
charged with negative electricity (and is denoted by 
the minus sign — ). These terms are, however, purely 
arbitrary, for in the present state of science we do not 
know which of these two states really means more and 



10 ELECTRICITY AND MAGNETISM part i 

which means less. In many ways electricity behaves as 
a weightless substance as incompressible as any material 
liquid. It is, however, quite certain that electricity is not 
a material fluid , whatever else it may be. For while it 
resembles a fluid in its property of apparently flowing 
from one point to another, it differs from every known 
fluid in almost every other respect. It possesses no 
weight; it repels itself. It is, moreover, quite impossible 
to conceive of two fluids whose properties should in every 
respect be the precise opposites of one another. For 
these reasons it is clearly misleading to speak of an 
electric fluid or fluids, however convenient the term may 
seem to be. In metals and other good conductors elec- 
tricity can apparently move and flow quite easily in 
currents. In transparent solids, such as glass and resin, 
and in many transparent liquids such as oils, and in 
gases such as the air (if still, and not rarefied) electricity 
apparently cannot flow. Even a vacuum appears to be a 
non-conductor. In the case of all non-conductors elec- 
tricity can only be moved by an action known as displace- 
ment (see Art. 57). 

It appears then that in metals electricity can easily 
pass from molecule to molecule ; but in the case of non- 
conductors the electricity is in some way stuck to the 
molecules, or associated with them. Some electricians, 
notably Faraday, have propounded a molecular theory 
of electricity, according to which the electrical states are 
the result of certain peculiar conditions of the molecules 
of the surfaces that have been rubbed. Another view is 
to regard the state of electrification as related to the ether 
(the highly-attenuated medium which fills all space, and 
is the vehicle by which light is transmitted), which is 
known to be associated with the molecules of matter. 
Some indeed hold that the ether itself is electricity ; and 
that the two states of positive and negative electrifica- 
tion are simply due to displacement of the ether at the 
surfaces of bodies. In these lessons we shall avoid as 



CHAP. I 



ELECTRIC CHARGES 



11 



far as possible all theories, and shall be content to use 
the term electricity. 

8. Charge. — The quantity of electrification of either 
kind produced by friction or other means upon the surface 
of a body is spoken of as a charge, and a body when 
electrified is said to be charged. It is clear that there 
may be charges of different values as well as of either 
kind. When the charge of electricity is removed from 
a charged body it is said to be discharged. Good con- 
ductors of electricity are instantaneously discharged if 
touched by the hand or by any conductor in contact with 
the ground, the charge thus finding a means of escaping 
to earth or to surrounding walls. A body that is not a 
good conductor may be readily discharged by passing it 
rapidly through the flame of a spirit-lamp or a candle ; 
for the hot gases instantly carry off the charge and dis- 
sipate it in the air. 

Electricity may either reside upon the surface of bodies 
as a charge, or flow through their substance as a current. 
That branch of the science which treats of the laws of the 
charges, that is to say, of electricity at rest, upon the 
surface of bodies is termed electrostatics, and is dealt 
with in Chapter IV. The branch of the subject which 
treats of the flow of electricity in currents is dealt with 
in Chapter III., and other later portions of this book. 

9. Modes of representing Electrification. — Several 
modes are used to represent the electrification of surfaces. 
In Figs. 6, 7, and 8 are rep- 



A B 



B 



resented two disks — A cov- 
ered with woollen cloth, B 
of some resinous body, — 
which have been rubbed to- 
gether so that A has become 
positively, B negatively elec- 
trified. In Fig. 6 the sur- 
faces are marked with plus ( + ) and minus ( 



B 



Fig. 6. 



Fig. 7. 



Fig. S. 

-) signs. 



In Fig. 7 dotted lines are drawn just outside the posi- 



12 ELECTRICITY AND MAGNETISM part i 

tively electrified surface and just within the negatively 
electrified surface, as though one had a surplus and' the 
other a deficit of electricity. In Fig. 8 lines are drawn 
across the intervening space from the positively electrified 
surface to the opposite negative charge. The advantages 
of this last mode are explained in Art. 13. 

10. Conductors and Insulators. — The term "con- 
ductors," used above, is applied to those bodies which 
readily allow electricity to flow through them. Roughly 
speaking, bodies may be divided into two classes — those 
which conduct and those which do not; though very 
many substances are partial conductors, and cannot well 
be classed in either category. All the metals conduct 
well; the human body conducts, and so does water. On 
the other hand glass, sealing-wax, silk, shellac, gutta- 
percha, indiarubber, resin, fatty substances generally, 
and the air, are non-conductors. On this account these 
substances are used to make supports and handles for 
electrical apparatus where it is important that the elec- 
tricity should not leak away ; hence they are sometimes 
called insulators or isolatois. Faraday termed them dielec- 
trics. We have remarked above that the name of non- 
electrics was given to those substances which, like the 
metals, yield no sign of electrification when held in the 
hand and rubbed. We now know the reason why they 
show no electrification ; for, being good conductors, the 
electrification flows away as fast as it is generated. The 
observation of Gilbert that electrical experiments fail 
in damp weather is also explained by the knowledge that 
water is a conductor, the film of moisture on the surface 
of damp bodies causing the electricity produced by friction 
to leak away as fast as it is generated. 

11. Other Electrical Effects. — The production of elec- 
tricity by friction is attested by other effects than those 
of attraction and repulsion, which hitherto w r e have 
assumed to be the test of the presence of electricity. 
Otto von Guericke first observed that sparks and flashes 



chap, i SOURCES OF ELECTRIFICATION 13 

of light could be obtained from highly electrified bodies 
at the moment when they were discharged. Such sparks 
are usually accompanied by a snapping sound, suggesting 
on a small scale the thunder accompanying the lightning 
spark, as was remarked by New ton and other early 
observers. Pale flashes of light are also produced by the 
discharge of electricity through tubes partially exhausted 
of air by the air-pump. Other effects will be noticed in 
due course. 

12. Other Sources of Electrification. — The student 
must be reminded that friction is by no means the only 
source of electrification. The other sources, percussion, 
compression, heat, chemical action, physiological action, 
contact of metals, etc., will be treated of in Lesson VII. 
We will simply remark here that friction between two 
different substances always produces electrical separa- 
tion, no matter what the substances may be. Symmer 
observed the production of electrification when a silk 
stocking was drawn over a woollen one, though woollen 
rubbed upon woollen, or silk rubbed upon silk, produces 
no electrical effect. If, however, a piece of rough glass 
be rubbed on a piece of smooth glass, electrification is 
observed ; and indeed the conditions of the surface play 
a very important part in the production of electrification 
by friction. In general, of two bodies thus rubbed 
together, that one becomes negatively electrical whose 
particles are the more easily removed by friction. Differ- 
ences of temperature also affect the electrical conditions 
of bodies, a warm body being usually negative when 
rubbed on a cold piece of the same substance. The 
quantity of electrification produced is, however, not pro- 
portional to the amount of the actual mechanical friction ; 
hence it appears doubtful whether friction is truly the 
cause of the electrification. Something certainly happens 
when the surfaces of two different substances are brought 
into intimate contact, which has the result that when 
they are drawn apart they are found (provided at least 



14 ELECTRICITY AND MAGNETISM part i 

one of them is a non-conductor) to have acquired opposite 
charges of electrification; one surface having apparently 
taken some electricity from the other. But these opposite 
charges attract one another and cannot be drawn apart 
without there being mechanical work done upon the 
system. The work thus spent is stored up in the act 
of separating the charged surfaces; and as long as 
the charges remain separated they constitute a store 
of potential energy. The so-called frictional electric 
machines are therefore machines for bringing dissimilar 
substances into intimate contact, and then drawing apart 
the particles that have touched one another and become 
electrical. 

If the two bodies that are rubbed together are both 
good conductors, they will not become strongly electrified, 
even if held on insulating handles. It is quite likely, 
however, that the heat produced by friction, as in the 
bearings of machinery, is due to electric currents gen- 
erated where the surfaces meet and slip. 

13. Electric Field. — Whenever two oppositely 
charged surfaces are placed near one another they tend 
to move together, and the space between them is found 
to be thrown into a peculiar state of 
stress, as though the medium in between 
had been stretched. To explore the 
space between two bodies one of which 
has been positively and the other nega- 
tively electrified, we may use a light 
pointer (Fig. 9) made of a small piece of very thin paper 
pierced with a hole through w T hich passes a long thread 
of glass. It will be found that this pointer tends to 
point across from the positively electrified surface to 
the negatively electrified surface, along invisible lines of 
electric force. The space so filled with electric lines of 
force is called an electric field. In Fig. 8 A and B 
represent two bodies the surfaces of which have been 
electrified, the one positively, the other negatively. In 




ELECTROSCOPES 



15 



the field between them the electric lines pass across 

almost straight, except near the edges, where they are 

curved. Electric lines of force start from a positively 

charged surface at one end, and 

end on a negatively charged 

surface at the other end. They 

never meet or cross one another. 

Their direction indicates that of 

the resultant electric force at 

every point through which they 

pass. The stress in the medium 

thus mapped out by the lines of 

force acts as a tension along 

them, as though they tended to 

shorten themselves. In fact in Fig. 8 the tension 

medium draws the two surfaces together. There 

a pressure in the medium at right angles to the lines, 

tending to widen the distance between them. Fig. 10 

represents a ball which has been positively electrified, 

and placed at a distance from other objects; the lines in 

the field being simply radial. 




Fig. 10. 



in the 
is also 



Lesson II. — Electroscopes 

14. Simple Electroscopes. — An instrument for detect- 
ing whether a body is electrified or not, and whether 
the electrification is positive or negative, is termed an 
Electroscope. The feather which was attracted or re- 
pelled, and the two pith-balls which flew apart, as we 
found in Lesson I., are in reality simple electroscopes. 
There are, however, a number of pieces of apparatus 
better adapted for this particular purpose, some of which 
we will describe. 

15. Needle Electroscope. — The earliest electroscope 
was that devised by Dr. Gilbert, and shown in Fig. 11, 
which consists of a stiff strip balanced lightly upon a 
sharp point. A thin strip of brass or wood, a straw, or 



16 



ELECTRICITY AND MAGNETISM part i 



even a goose quill, balanced upon a sewing needle, will 
serve equally well. When an electrified body is held near 




Fig. 11. 

the electroscope it is attracted and turned round, and will 
thus indicate the presence of electric charges far too feeble 
to attract bits of paper from a table. 

16. Gold-Leaf Electroscope. — A still more sensi- 




Fig. u 



tive instrument is the Gold-Leaf Electroscope, invented 
by Ben net, and shown in Fig. 12. We have seen 
how two pith-balls when similarly electrified repel one 



chap, i GOLD-LEAF ELECTROSCOPE 17 

another and stand apart, gravity being partly overcome 
by the force of the electric repulsion. A couple of 
narrow strips of the thinnest tissue paper, hung upon a 
support, will behave similarly when electrified. But the 
best results are obtained with two strips of gold leaf, 
which, being excessively thin, is much lighter than the 
thinnest paper. The Gold -Leaf Electroscope is con- 
veniently made by suspending the two leaves within a 
wide-mouthed glass jar, which both serves to protect 
them from draughts of air and to support them from 
contact with the ground. The mouth of the jar should 
be closed by a plug of paraffin wax, through which is 
pushed a bit of varnished glass tube. Through this 
passes a stiff brass wire, the lower end of which is bent 
at a right angle to receive the two strips of gold leaf, 
while the upper supports a flat plate of metal, or may be 
furnished with a brass knob. AVhen kept dry and free 
from dust it will indicate excessively small quantities of 
electrification. A rubbed glass rod, even while two or 
three feet from the instrument, will cause the leaves to 
repel one another. The chips produced by sharpening a 
pencil, falling on the electroscope top, are seen to be 
electrified. If the knob be even brushed with a small 
camel's hair brush, the slight friction produces a percep- 
tible effect. With this instrument all kinds of friction 
can be shown to produce electrification. Let a person, 
standing upon an insulating support, — such as a stool 
with glass legs, or a board supported on four glass 
tumblers, — be briskly struck with a silk handkerchief, 
or with a fox's tail, or even brushed with a clothes brush, 
he will be electrified, as will be indicated by the electro- 
scope if he place one hand on the knob at the top of it. 
The Gold-Leaf Electroscope can further be used to indi- 
cate the kind of electrification on an excited body. Thus, 
suppose we rubbed a piece of brown paper with a piece of 
indiarubber and desired to find out whether the electri- 
fication excited on the paper was + or — , we should 
c 



18 



ELECTRICITY AND MAGNETISM part i 



proceed as follows : — First charge the gold leaves of the 
electroscope by touching the knob with a glass rod rubbed 
on silk. The leaves diverge, being electrified with -f 
electrification. When they are thus charged the approach 
of a body which is positively electrified will cause them 
to diverge still more widely ; while, on the approach of 
one negatively electrified, they will tend to close together. 
If now the brown paper be brought near the electroscope, 
the leaves will be seen to diverge more, proving the 
electrification of the paper to be of the same kind as 
that with which the electroscope is charged, or positive. 

Sometimes the outer surface 
of the glass jar containing 
the gold leaves is covered 
with wire gauze or strips of 
foil to shield the leaves from 
the influence of external 
bodies. A preferable way is 
to use glass of a kind that 
conducts. 

The part played by the 
surrounding medium in the 
operation of the electroscope 
is illustrated by Fig. 13. 
Of the electric lines in the 
field surrounding the rubbed rod a number will pass into 
the metal cap of the electroscope and emerge below 
through the leaves. The nearer the rod is brought, the 
greater will be the number of electric lines thus affecting 
the instrument. There being a tension along the lines 
and a pressure across them, the effect is to draw the gold 
leaves apart as though they repelled each other. 

The Gold-Leaf Electroscope will also indicate roughly 
the amount of electrification on a body placed in contact 
with it, for the gold leaves open out more widely when 
the charge thus imparted to them is greater. For exact 
measurement, however, of the degree of electrification, 




Fi£. lo 



ELECTROSCOPE 



19 



recourse must be had to the instruments known as 
Electrometers, described in Lesson XXII. 

In another form of electroscope (Bohnenberger's) a 
single gold leaf is used, and is suspended between two 
metallic plates, one of which can be positively, the other 
negatively electrified, by placing them in communication 
with the poles of a "dry pile" (Art. 193). If the gold 
leaf be charged positively or negatively it will be attracted 
to one side and repelled from the other, according to the 
law of attraction and repulsion mentioned in Art. 4. 

17. Henley's Semaphore. — As an indicator for large 
charges of electricity there is sometimes used a sema- 
phore like that shown in Fig. 14. 

It consists of a pith-ball at the end 
of a light arm fixed on a pivot to 
an upright. When the whole is 
electrified the pith-ball is repelled 
from the upright and flies out at an 
angle, indicated on a graduated 
scale or dial behind it. This little 
electroscope, which is seldom used 
except to show whether an electric 
machine or a Leyden battery is 
charged, must on no account be con- 
fused with the delicate " Quadrant 
Electrometer " described in Lesson 
XXIL, whose object is to measure very small charges of 
electricity — not to indicate large ones. 

18. The Torsion Balance. — Although more properly 
an Electrometer than a mere Electroscope, it will be 
most convenient to describe here the instrument known 
as the Torsion Balance (Fig. 15). This instrument, once 
famous, but now quite obsolete, served to measure the 
force of the repulsion between two similarly electrified 
bodies, by balancing the repelling force against the force 
exerted by a fine wire in untwisting itself after it has 
been twisted. The torsion balance consists of a light arm 




Fig. 14. 



20 



ELECTRICITY AND MAGNETISM part i 



or lever of shellac suspended within a cylindrical glass 
case by means of a fine silver wire. At one end this 
lever is furnished with a gilt pith-ball n. The upper 
end of the silver wire is fastened to a brass top, upon 

which a circle, divided 
into degrees, is cut. This 
top can be turned round 
in the tube which sup- 
ports it, and is called the 
torsion-head. Through an 
aperture in the cover there 
can be introduced a sec- 
ond gilt pith-ball ??i, fixed 
to the end of a vertical 
glass rod a. Round the 
glass case, at the level of 
the pith-balls, a circle is 
drawn, and divided also 
into degrees. 
Fiff * 15, In using the torsion 

balance to measure the amount of a charge of electricity, 
the following method is adopted : — First, the torsion-head 
is turned round until the two pith-balls m and n just 
touch one another. Then the glass rod a is taken out, 
and the charge of electricity to be measured is imparted 
to the ball m, which is then replaced in the balance. As 
soon as m and n touch one another, part of the charge 
passes from m to n, and they repel one another because 
they are then similarly electrified. The ball ??, therefore, 
is driven round and twists the wire up to a certain extent. 
The force of repulsion becomes less and less as n gets 
farther and farther from m ; but the force of the twist 
gets greater and greater the more the wire is twisted. 
Hence these two forces will balance one another when 
the balls are separated by a certain distance, and it is 
clear that a large charge of electricity will repel the ball 
n with a greater force than a lesser charge would. The 




chap, i LAW OF INVERSE SQUARES 21 

distance through which the ball is repelled is read off in 
angular degrees of the scale. When a wire is twisted, 
the force with which it tends to untwist is precisely pro- 
portional to the amount of the twist. The force required 
to twist the wire ten degrees is just ten times as great 
as the force required to twist it one degree. In other 
words, the force of torsion is proportional to the angle of 
torsion. The angular distance between the two balls is, 
when they are not very widely separated, very nearly 
proportional to the actual straight distance between them, 
and represents the force exerted between electrified balls 
at that distance apart. The student must, however, care- 
fully distinguish between the measurement of the force 
and the measurement of the actual quantity of electricity 
with which the instrument is charged. For the force 
exerted between the electrified balls will vary at different 
distances according to a particular law known as the 
"law of inverse squares," which requires to be carefully 
explained. 

19. The Law of Inverse Squares. — Coulomb proved, 
by means of the Torsion Balance, that the force exerted 
between two small electrified bodies varies inversely as 
the square of the distance between them when the 
distance is varied. Thus, suppose two small electrified 
bodies 1 inch apart repel one another with a certain 
force, at a distance of 2 inches the force will be found 
to be only one quarter as great as the force at 1 inch ; 
and at 10 inches it will be only t £q part as great as 
at 1 inch. This law is proved by the following ex- 
periment with the torsion balance. The two scales were 
adjusted to 0°, and a certain charge was then imparted 
to the balls. The ball n was repelled round to a distance 
of 36°. The twist on the wire between its upper and 
low T er ends was also 36°, or the force tending to repel 
was thirty-six times as great as the force required to 
twist the wire by 1°. The torsion-head was now turned 
round so as to twist the thread at the top and force 



22 ELECTRICITY AND MAGNETISM part i 

the ball n nearer to m, and was turned round until 
the distance between n and m was halved. To bring 
down this distance from 36° to 18°, it was found 
needful to twist the torsion-head through 126°. The 
total twist between the upper and lower ends of the 
wire was now 126°+ 18°, or 144°; and the force was 
144 times as great as that force which would twist 
the wire 1°. But 144 is four times as great as 36 ; 
hence we see that while the distance had been reduced 
to one half] the force between the balls had become 
four times as great. Had we reduced the distance 
to one quarter, or 9°, the total torsion would have been 
found to be 576°, or sixteen times as great; proving 
the force to vary inversely as the square of the dis- 
tance. 

In practice it requires great experience and skill to 
obtain results as exact as this, for there are many sources 
of inaccuracy in the instrument. The balls must be very 
small, in proportion to the distances between them. The 
charges of electricity on the balls are found, moreover, to 
become gradually less and less, as if the electricity leaked 
away into the air. This loss is less if the apparatus be 
quite dry. It is therefore usual to dry the interior by 
placing inside the case a cup containing either chloride 
of calcium, or pumice stone soaked with strong sulphuric 
acid, to absorb the moisture. 

Before leaving the subject of electric forces, it may be 
well to mention that the force of attraction between two 
oppositely electrified bodies varies also inversely as the 
square of the distance between them. And in every 
case, whether of attraction or repulsion, the force at any 
given distance is proportional to the product of the two 
quantities of electricity on the bodies. Thus, if we 
had separately given a charge of 2 to the ball m and a 
charge of 3 to the ball n, the force between them will be 
3x2 = 6 times as great as if each had had a charge of 1 
given to it. It must be remembered, however, that the 




chap, i ELECTRIC FIELD 23 

law of inverse squares is only true when applied to the 
case of bodies so small, as compared with the distance 
between them, that they are mere points. For flat, large, 
or elongated bodies the law of inverse squares does not 
hold good. The attraction between two large flat disks 
oppositely electrified with given charges, and placed near 
together, does not vary with the distance. 

20. Field between two Balls. — The electric field 
(Art. 13) between two oppositely electrified balls is found 
to consist of curved lines. 
By the principle laid down 
in Art. 13, there is a tension 
along these lines so that 
they tend not only to draw 
the two balls together, but 
also to draw the electrifica- 
tions on the surfaces of the 
balls toward one another. Fl £* 16 - 

There is also a lateral pressure in the medium tending to 
keep the electric lines apart from one another. One 
result of these actions is that the charges are no longer 
equally distributed over the surfaces, but are more dense 
on the parts that approach most nearly. 

21. Unit Quantity of Electricity. — In consequence of 
these laws of attraction and repulsion, it is found most 
convenient to adopt the following definition for that 
quantity of electricity which we take for a unit or stand- 
ard by which to measure other quantities of electricity. 
One {electrostatic) Unit of Electricity is that quantity which, 
when placed at a distance of one centimetre in air from 
a similar and equal quantity, repels it icith a force of one 
dyne. If instead of air another medium occupies the 
space, the force will be different. For example, if petro- 
leum is used the force exerted between given charges 
will be about half as great (see Art. 56). Further in- 
formation about the measurement of electrical quantities 
is given in Lessons XXI. and XXII. 



24 



ELECTRICITY AND MAGNETISM part i 



Lesson III. — Electrification by Influence 

22. Influence. — We have now learned how two 
charged bodies may apparently attract or repel one 
another. It is sometimes said that it is the charges in 
the bodies which attract or repel one another; but as 
electrification is not known to exist except in or on 
material bodies, the proof that it is the charges them- 
selves which are acted upon is only indirect. Nevertheless 
there are certain matters which support this view, one of 




Fig. 17. 

these being the electric influence exerted by an electrified 
body upon one not electrified. 

Suppose we electrify positively a ball C, shown in Fig. 
17, and hold it near to a body that has not been electri- 
fied, what will occur? We take for this experiment the 
apparatus shown on the right, consisting of a long sausage- 
shaped piece of metal, either hollow or solid, held upon a 
glass support. This " conductor," so called because it is 
made of metal which permits electricity to pass freely 
through it or over its surface, is supported on glass to 



INFLUENCE 25 



prevent the escape of electricity to the earth, glass being 
a non-conductor. The influence of the positive charge 
of the ball placed near this conductor is found to induce 
electrification on the conductor, which, although it has 
not been rubbed itself, will be found to behave at its two 
ends as an electrified body. The ends of the conductor 
will attract little bits of paper; and if pith-balls be hung 
to the ends they are found to be repelled. It will, how- 
ever, be found that the middle region of the long-shaped 
conductor will give no sign of any electrification. Further 
examination will show that the two electrifications on the 
ends of the conductor are of opposite kinds, that nearest 
the excited glass ball being a negative charge, and that at 
the farthest end being an equal charge, but of positive 
sign. It appears then that a positive charge attracts 
negative and repels positive, and that this influence can 
be exerted at a distance from a body. If we had begun 
with a charge of negative electrification upon a stick of 
sealing-wax, the presence of the negative charge near the 
conductor would have induced a positive charge on the 
near end, and negative on the far end. This action, 
discovered in 1753 by John Canton, is spoken of as 
influence or electrostatic induction.* It will take 
place across a considerable distance. Even if a large 
sheet of glass be placed between, the same effect will be 
produced. When the electrified body is removed both 
the charges disappear and leave no trace behind, and 
the glass ball is found to be just as much electrified as 
before ; it has parted with none of its own charge. It 



* The word induction originally used was intended to denote an action 
at a distance, as distinguished from conduction, which implied the convey- 
ance of the action by a material conductor. But there were discovered 
other actions at a distance, namely, the induction of currents by moving 
magnets, or by other currents, and the induction of magnetism in iron in 
the presence of a neighbouring magnet. As the term induction has now 
been officially adopted for the induction of currents, its use in other senses 
ought to be dropped. Hence the preference now given to the term influ- 
ence for the induction of charges by charges. 



26 



ELECTRICITY AND MAGNETISM part i 



will be remembered that on one theory a body charged 
positively is regarded as having more electricity than 
the things round it, while one with a negative charge is 
regarded as having less. According to this view it would 
appear that when a body (such as the + electrified glass 
ball) having more electricity than things around it is 
placed near an insulated conductor, the uniform distribu- 
tion of electricity in that conductor is disturbed, the 
electricity flowing away from that end which is near the 
4- body, leaving less than usual at that end, and producing 

more than usual at the other 
end. This view of things will 
account for the disappear- 
ance of all signs of electrifi- 
cation when the electrified 
body is removed, for then 
the conductor returns to its 
former condition ; and being 
neither more nor less elec- 
trified than all the objects 
around on the surface of the 
earth, will show neither positive nor negative charge. 
The action is not, however, a mere action at a distance; 
it is one in which the intervening medium takes an essen- 
tial part. Consider (Fig. 18) what takes place when an 
insulated, non-electrified metal ball B is brought under 
the influence of a positively electrified body A. At 
once some of the electric lines of the field that surrounds 
A pass through B, entering it at the side nearer A, and 
leaving it at the farther side. As the ball B has no 
charge of its own, as many electric lines will enter on one 
side as leave on the other; or, in other words, the induced 
negative charge on one side and the induced positive 
charge on the other will be exactly equal in amount. 
They will not, however, be quite equally distributed, the 
negative charge on the side nearer A being more concen- 
trated, and the lines in the field on that side denser. 




chap, i ELECTRIC INFLUENCE 27 

23. Effects of Influence. — If the conductor be made 
in two parts, which while under the influence of the 
electrified body are separated, then on the removal of the 
electrified body the two charges can no longer return to 
neutralize one another, but remain each on its own 
portion of the conductor. 

If the conductor be not insulated on glass supports, 
but placed in contact with the ground, that end only 
which is nearest the electrified body will be found to 
be electrified. The repelled charge is indeed repelled as 
far as possible into the walls of the room ; or, if the 
experiment be performed in the open air, into the earth. 
One kind of electrification only is under these circum- 
stances to be found, namely, the opposite kind to that 
of the excited body, whichever this may be. The same 
effect occurs in this case as if an electrified body had the 
power of attracting up the opposite kind of charge out of 
the earth. 

The quantity of the two charges thus separated by 
influence on such a conductor in the presence of a charge 
of electricity, depends upon the amount of the charge, 
and upon the distance of the charged body from the 
conductor. A highly electrified glass rod will exert a 
greater influence than a less highly electrified one ; and 
it produces a greater effect as it is brought nearer and 
nearer. The utmost it can do will be to induce on the 
near end a negative charge equal in amount to its own 
positive charge, and a similar amount of positive electri- 
fication at the far end ; but usually, before the electrified 
body can be brought so near as to do this, something else 
occurs which entirely alters the condition of things. As 
the electrified body is brought nearer and nearer, the 
charges of opposite sign on the two opposed surfaces 
attract one another more and more strongly and accumu- 
late more and more densely, until, as the electrified body 
approaches very near, a spark is seen to dart across, the 
two charges thus rushing together to neutralize one 



28 



ELECTRICITY AND MAGNETISM part i 




another, leaving the induced charge of positive electricity, 
which was formerly repelled to the other end of the 
conductor, as a permanent charge after the electrified 
body has been removed. 

In Fig. 19 is illustrated the operation of gradually 
lowering down over a table a positively electrified metal 
ball. The nearer it approaches the table, the more does 
the electric field surrounding it concentrate itself in the 
gap between the ball and the 
table top; the latter becoming 
negatively electrified by influ- 
ence. Where the electric lines 
are densest the tension in the 
medium is greatest, until when 
the ball is lowered still further 
the mechanical resistance of the 
air can no longer withstand 
the stress; it breaks down and 
the layer of air is pierced by a 
spark. If oil is used as a surrounding medium instead of 
air, it will be found to stand a much greater stress without 
being pierced. 

24. Attraction due to Influence. — We are now able 
to apply the principle of influence to explain why an 
electrified body should attract things that have not been 
electrified at all. Fig. 18, on p. 26, may be taken to 
represent a light metal ball B hung from a silk thread 
presented to the end of a rubbed glass rod A. The 
positive charge on A produces by influence a negative 
charge on the nearer side of B and an equal positive 
charge on the far side of B. The nearer half of the ball 
will therefore be attracted, and the farther half repelled ; 
but the attraction will be stronger than the repulsion, 
because the attracted charge is nearer than the repelled. 
Hence on the whole the ball will be attracted. It can 
easily be observed that if a ball of non-conducting 
substance, such as wax, be employed, it is not attracted 



Pig. id. 



chap, i THE ELECTROPHORUS 29 

so much as a ball of conducting material. This in itself 
proves that influence really precedes attraction. 

Another way of stating the facts is as follows : — The 
tension along the electric field on the right of B will be 
greater than that on the left, because of the greater 
concentration of the electric lines on the right. 

25. Dielectric Power. — We have pointed out several 
times what part the intervening medium plays in these 
actions at a distance. The air, oil, glass, or other material 
between does not act simply as a non-conductor; it takes 
part in the propagation of the electric forces. Hence 
Faraday, who discovered this fact, termed such materials 
dielectrics. Had oil, or solid sulphur, or glass been used 
instead of air, the influence exerted by the presence of the 
electrified body at the same distance would have been 
greater. The power of a non-conducting substance to 
convey the influence of an electrified body across it is 
called its dielectric power (or was formerly called its 
specific inductive capacity, see Art. 56 and Lesson XXIII.) . 

26. The Electrophorus. — We are now prepared to 
explain the operation of a simple and ingenious instru- 
ment, devised by Volta in 1775, for the purpose of 
procuring, by the principle of influence, an unlimited 
number of charges of electricity from one single charge. 
This instrument* is the Electrophorus (Fig. 20). It 
consists of two parts, a round cake of resinous material 
cast in a metal dish or "sole," about 12 inches in 
diameter, and a round disk of slightly smaller diameter 
made of metal, or of wood covered with tinfoil, and 
provided with a glass handle. Shellac, or sealing-wax, or 
a mixture of resin, shellac, and Venice turpentine, may 
be used to make the cake. A slab of sulphur will also 
answer, but it is liable to crack. Sheets of hard ebonized 
indiarubber are excellent ; but the surface of this substance 

* Volta's electrophorus was announced in 1775. Its principle had 
already been anticipated by Wilcke, who in 1762 described to the Swedish 
Academy of Sciences two " charging-machines "working by influence. 



30 



ELECTRICITY AND MAGNETISM part i 



requires occasional washing with ammonia and rubbing 
with paraffin oil, as the sulphur con tamed in it is liable 
to oxidize and to attract moisture. To use the electro- 
phorus the resinous cake must be beaten or rubbed with 
a warm piece of woollen cloth, or, better still, with a cat's 




Fig. 20. 

skin. The disk or " cover " is then placed upon the cake, 
touched momentarily with the ringer, then removed by 
taking it up by the glass handle, when it is found to be 
powerfully electrified with a positive charge, so much so 
indeed as to yield a spark when the knuckle is presented 
to it. The " cover " may be replaced, touched, and once 
more removed, and will thus yield any number of sparks, 



chap, i THE ELECTROPHORUS 31 

the original charge on the resinous plate meanwhile 
remaining practically as strong as before. 

The theory of the elect rophor us is very simple, pro- 
vided the student has clearly grasped the principle of 
influence explained above. When the resinous cake is 
first beaten with the cat's skin its surface is negatively 
electrified, as indicated in Fig. 21. When the metal disk 
is placed down upon it, it rests really only on three or 
four points of the surface, and may be regarded as an 
insulated conductor in the presence of an electrified body. 
The negative electrification of the cake therefore acts by 
influence on the metallic disk or " cover," the natural 
electricity in it being displaced downwards, producing a 
positive charge on the under side, and leaving the upper 



\JT \ \ 



jyrrv-j.Ywx, 



TTrnrn \ TY-'r 

Fig. 21. Fig. 22. 

side negatively electrified. This state of things is shown 
in Fig. 22. If now the cover be touched for an instant 
with the finger, the negative charge of the upper surface 
will be neutralized by electricity flowing in from the earth 
through the hand and body of the experimenter. The 
attracted positive charge will, however, remain, being 
bound as it were by its attraction towards the negative 
charge on the cake. Fig. 23 shows the condition of 
things after the cover has been touched. If, finally, the 
cover be lifted by its handle, the remaining positive 
charge will be no longer "bound" on the lower surface 
by attraction, but will distribute itself on both sides of 
the cover, and may be used to give a spark, as already 
said. It is clear that no part of the original charge has 
been consumed in the process, which may be repeated as 



32 



ELECTRICITY AND MAGNETISM part i 



often as desired. As a matter of fact, the charge on the 
cake slowly dissipates — especially if the air be damp. 
Hence it is needful sometimes to renew the original charge 
by afresh beating the cake with the cat's skin. The 
labour of touching the cover with the finger at each 
operation may be saved by having a pin of brass or a 
strip of tinfoil projecting from the metallic "sole " on to 
the top of the cake, so that it touches the plate each time, 
and thus neutralizes the negative charge by allowing 
electricity to flow in from the earth. 

The principle of the electrophorus may then be 
summed up in the following sentence. A conductor if 



s 



Fig. 23. 




rr -rrrri 



Fig. U. 



touched while under the influence of a charged body acquires 
thereby a charge of opposite sign.* 

Since the electricity thus yielded by the electro- 
phorus is not obtained at the expense of any part of the 
original charge, it is a matter of some interest to inquire 
what the source is from which the energy of this apparently 
unlimited supply is drawn ; for it cannot be called into 

* Priestley, in 1T67, stated this principle in the following language : — 
" The electric fluid, when there is a redundancy of it in any body, repels 
the electric fluid in any other body, when they are brought within the 
sphere of each other's influence, and drives it into the remote parts of the 
body ; or quite out of the body, if there be any outlet for that purpose. 
In other words, bodies imuierged in electric atmospheres always become 
possessed of the electricity, contrary to that of the body, in whose atmo- 
sphere they are immerged." 



chaf. i FREE AND BOUND CHARGES 33 

existence without the expenditure of some other form of 
energy, any more than a steam-engine can work without 
fuel. As a matter of fact it is found that it is a little 
harder work to lift up the cover when it is charged than 
if it were not charged ; for, when charged, there is the 
tension of the electric field to be overcome as well as the 
force of gravity. Slightly harder work is done at the ex. 
pense of the muscular energies of the operator; and this 
is the real origin of the energy stored up in the separate 
charges. The purely mechanical actions of putting down 
the disk on the cake, touching it, and lifting it up, 
can be performed automatically by suitable mechanical 
arrangements, which render the production of these 
inductive charges practically continuous. Of such con- 
tinuous electrophori, the latest is Wimshurst's machine, 
described in Lesson V. 

27. "Free" and " Bound" Electrification. — We 
have spoken of a charge of electricity on the surface of a 
conductor, as being " bound " when it is attracted by the 
presence of a neighbouring charge of the opposite kind. 
The converse term " free " is sometimes applied to the 
ordinary state of electricity upon a charged conductor, 
not in the presence of a charge of an opposite kind. A 
"free" charge upon an insulated conductor flows away 
instantaneously to the earth, if a conducting channel be 
provided, as will be explained. It is immaterial what 
point of the conductor be touched. Thus, in the case 
represented in Fig. 17, wherein a -f electrified body 
induces — electrification at the near end. and — electri- 
fication at the far end of an insulated conductor, the — 
charge is " bound," being attracted, while the -f charge 
at the other end. being repelled, is " free " ; and if the 
insulated conductor be touched by a person standing on the 
ground, the "free" charge will flow away through his body 
to the earth, or to the walls of the room, while the " bound " 
charge will remain, no matter whether he touch the con- 
ductor at the far end. or at the near end, or at the middle. 



34 ELECTRICITY AND MAGNETISM part i 

28. Method of charging the Gold-Leaf Electroscope 
by Influence. — The student will now be prepared to 
understand the method by which a Gold-Leaf Electro- 
scope can be charged with the opposite kind of charge to 
that of the electrified body used to charge it. In Lesson 
II. it was assumed that the way to charge an electroscope 
was to place the excited body in contact with the knob, 
and thus permit, as it were, a small portion of the charge 
to flow into the gold leaves. A rod of glass rubbed on 
silk being + would thus obviously impart -f electrifica- 
tion to the gold leaves. 

Suppose, however, the rubbed glass rod to be held a 
few inches above the knob of the electroscope, as is 
indeed shown in Fig. 12. Even at this distance the gold 
leaves diverge, and the effect is due to influence. The 
gold leaves, and the brass wire and knob, form one con- 
tinuous conductor, insulated from the ground by the 
glass jar. The presence of the -f charge of the glass acts 
inductively on this "insulated conductor," inducing — 
electrification on the near end or knob, and inducing + 
at the far end, i.e. on the gold leaves, which diverge. 
Of these two induced charges, the — on the knob is 
"bound," while the + on the leaves is "free." If now, 
while the excited rod is still held above the electroscope, 
the knob be touched by a person standing on the ground, 
one of these two induced charges flows to the ground, 
namely, the free charge — not that on the knob itself, for 
it was "bound," but that on the gold leaves which was 
"free" — and the gold leaves instantly drop down straight. 
There now remains only the — charge on the knob, 
" bound " so long as the + charge of the glass rod is 
near to attract it. But if, finally, the glass rod be taken 
right away, the — charge is no longer "bound" on the 
knob, but is "free" to flow into the leaves, which once 
more diverge — but this time with a negative electrification. 

29. The " Return-Shock." — It is sometimes noticed 
that, when a charged conductor is suddenly discharged, 



CONDUCTION 35 



a discharge is felt by persons standing near, or may 
even affect electroscopes, or yield sparks. This action, 
known as the " return-shock," is due to influence. For 
in the presence of a charged conductor a charge of 
opposite sign will be induced in neighbouring bodies, 
and on the discharge of the conductor these neighbour- 
ing bodies may also suddenly discharge their induced 
charge into the earth, or into other conducting bodies. 
A "return-shock" is sometimes felt by persons standing 
on the ground at the moment when a flash of lightning 
has struck an object some distance away. 



Lesson IV. — Conduction and Distribution of Electricity 

30. Conduction. — Toward the close of Lesson I. we 
explained how certain bodies, such as the metals, conduct 
electricity, while others are non-conductors or insulators. 
This discovery is due to Stephen Gray ; who. in 1729, 
found that a cork, inserted into the end of a rubbed glass 
tube, and even a rod of wood stuck into the cork, pos- 
sessed the power of attracting light bodies. He found, 
similarly, that metallic wire and pack-thread conducted 
electricity, while silk did not. 

We may repeat these experiments by taking (as in 
Fig. 25) a glass rod, fitted with a cork and a piece of 
vyood. If a bullet or a brass knob be hung to the end of 
this by a linen thread or a wire, it is found that when the 
glass tube is rubbed the bullet acquires the property of 
attracting light bodies. If a dry silk thread is used, 
however, no electricity will flow down to the bullet. 

Gray even succeeded in transmitting a charge of 
electricity through a hempen thread over 700 feet long, 
suspended on silken loops. A little later Du Fay 
succeeded in sending electricity to no less a distance 
than 1256 feet through a moistened thread, thus proving 
the conducting power of moisture. From that time the 



36 



ELECTRICITY AND MAGNETISM part i 



. 



classification of bodies into conductors and insulators has 
been observed. 

This distinction cannot, however, be entirely main- 
tained, as a large class of substances occupy an inter- 
mediate ground as partial conductors. For example, dry 
wood is a bad conductor and also a bad insulator ; it 
is a good enough conductor to conduct away the high- 
potential electricity obtained by friction, but it is a 
bad conductor for the relatively low-potential electricity 
of small voltaic batteries. Substances that are very bad 





Fig. 25. 






conductors are said to offer a great resistance to the 
flow of electricity through them. There is indeed no 
substance so good a conductor as to be devoid of resist- 
ance. There is no substance of so high a resistance as 
not to conduct a little. Even silver, which conducts best 
of all known substances, resists the flow of electricity to 
a small extent ; and, on the other hand, such a non-con- 
ducting substance as glass, though its resistance is many 
million times greater than any metal, does allow a very 
small quantity of electricity to pass through it. In the 



CONDUCTORS 



37 



following list, the substances named are placed in order, 
each conducting better than those lower down on the list. 



Silver . . 


1 


Copper . . 


i 


Other metals 


> Good Conductors. 


Charcoal . 


Water . . 


j 


The body . 


1 


Cotton . . 


j 


Dry wood . 


r Partial Conductors 


Marble . . 


1 


Paper . . 


j 


Oils . . . 


^ 


Porcelain . 




Wool . . 




Silk . . . 




Resin . . 




Guttapercha 


, Non-Conductors or 
j Insulators. 


Shellac . . 


Ebonite 




Paraffin 




Glass . . 




Quartz (fusee 


) 


Air . . . 


d 



A simple way of observing experimentally whether a 
body is a conductor or not, is to take a charged gold- 
leaf electroscope, and, holding the substance to be 
examined in the hand, touch the knob of the electro- 
scope w T ith it. If the substance is a conductor the elec- 
tricity will flow away through it and through the body 
to the earth, and the electroscope will be discharged. 
Through good conductors the rapidity of the flow is so 
great that the discharge is practically instantaneous. 
Further information on this question is given in Lesson 
XXXTIL 

31. Distribution of Charge on Bodies. — If electri- 
fication is produced at one part of a non-conducting 
body, it remains at that point and does not flow over 
the surface, or at most flows over it excessively slowly. 



38 ELECTRICITY AND MAGNETISM part i 

Thus if a glass tube is rubbed at oue end, only that one 
end is electrified. Hot glass is, however, a conductor. 
If a warm cake of resin be rubbed at one part with a 
piece of cloth, only the portion rubbed will attract light 
bodies, as may be proved by dusting upon it through 
a piece of muslin fine powders such as red lead, lyco- 
podium, or verdigris, which adhere where the surface is 
electrified. The case is, however, wholly different when 
a charge of electricity is imparted to any part of a con- 
ducting body placed on an insulating support, for it 
instantly distributes itself all over the surface, though in 
general not uniformly over all points of the surface. 

32. The Charge resides on the Surface. — A charge 
of electricity resides only on the surface of conducting 
bodies. This is proved by the fact that it is found 
to be immaterial to the distribution what the inte- 
rior of a conductor is made of: it may be solid metal, 
or hollow, or even consist of wood covered with tinfoil 
or gilt, but, if the shape be the same, the charge will 
distribute itself precisely in the same manner over the 
surface. There are also several ways of proving by 
direct experiment this very important fact. Let a hollow 
metal ball, having an aperture at the top. be taken (as in 
Fig. 26), and set upon an insulating stem, and charged 
by sending into it a few sparks from an electrophorus. 
The absence of any charge in the interior may be shown 
as follows : — In order to observe the nature of the elec- 
trification of a charged body, it is convenient to have some 
means of removing a small quantity of the charge as 
a sample for examination. To obtain such a sample, a 
little instrument known as a proof-plane is employed. 
It consists of a little disk of sheet copper or of gilt paper 
fixed at the end of a small glass rod. If this disk is laid 
on the surface of an electrified body at any point, part 
of the charge flows into it, and it may be then removed, 
and the sample thus obtained may be examined with a 
gold-leaf electroscope in the ordinary way. For some 



CHARGE ON SURFACE 



39 



purposes a metallic bead, fastened to the end of a glass 
rod, is more convenient than a flat disk. If such a proof- 
plane be applied to the outside of our electrified hollow 
ball, and then touched on the knob of an electroscope, 
the gold leaves will diverge, showing the presence of a 




Fig 



charge. But if the proof-plane be carefully inserted 
through the opening, and touched against the inside of 
the globe and then withdrawn, it will be found that 
the inside is destitute of electrification. An electrified 
pewter mug will show a similar result, and so will even 
a cylinder of gauze wire. 



40 



ELECTRICITY AND MAGNETISM part i 



33. Biotas Experiment. — Biot proved the same fact 
in another way. A copper ball was electrified and 
insulated. Two hollow hemispheres of copper, of a 
larger size, and furnished with glass handles, were then 
placed together outside it (Fig. 27). So long as they 
did not come into contact the charge remained on the 
inner sphere; but if the outer shell touched the inner 
sphere for but an instant, the whole of the charge passed 



A 




Fig. 27. 

to the exterior ; and when the hemispheres were separated 
and removed the inner globe was found to be completely 
discharged. 

34. Further Explanation. — Doubtless the explana- 
tion of this behaviour of electricity is to be found in the 
property previously noticed as possessed by either kind 
of electrification, namely, that of repelling itself ; hence 
it retreats as far as can be from the centre and remains 
upon the surface. An important proposition concerning 
the absence of electric force within a closed conductor is 
proved in Lesson XXI. ; meanwhile it must be noted that 
the proofs, so far, are directed to demonstrate the absence 



chap, i ELECTRIFICATION EXTERNAL 



41 



of a free charge of electricity in the interior of hollow con- 
ductors. Amongst other experiments, Terquem showed 
that a pair of gold leaves hung inside a wire cage could 
not be made to diverge when the cage was electrified. 
Faraday constructed a conical bag of linen-gauze, sup- 
ported as in Fig. 28, upon an insulating stand, and to 
which silk strings were attached, by which it could be 
turned inside out. It was charged, and the charge was 
shown by the proof -plane and electroscope to be on the 
outside of the bag. On turning it inside out the elec- 




Fig. 28. 



tricity was once more found outside. Faraday's most 
striking experiment was made with a hollow cube, 
measuring 12 feet each way, built of wood, covered with 
tinfoil, insulated, and charged with a powerful machine. 
so that large sparks and brushes were darting off from 
every part of its outer surface. Into this cube Faraday 
took his most delicate electroscopes ; but once within he 
failed to detect the least effect upon them. 

35. Applications. — Advantage is taken of this in 
the construction of delicate electrometers and other 



42 ELECTRICITY AND MAGNETISM part i 

instruments, which can be effectually screened from 
the influence of electrified bodies by enclosing them 
in a cover of thin metal, closed all round, except where 
apertures must be made for purposes of observation. 
Metal gauze answers excellently, and is nearly trans- 
parent. It was proposed by the late Professor Clerk 
Maxwell to protect buildings from lightning by covering 
them on the exterior with a network of wires. 

36. Apparent Exceptions. — There art 1 two apparent 
exceptions to the law that electrification resides only on 
the outside of conductors. (1) If there are electrified 
insulated bodies actually placed inside the hollow con- 
ductor, the presence of these electrified bodies acts 
inductively and attracts the opposite kind of charge to 
the inner side of the hollow conductor. ('2) When elec- 
tricity flows in a current, it flows through the substance 
of the conductor. The law is limited therefore to 
electricity at rest, — that is, to statical charges. 

37. Faraday's " Ice-pail " Experiment. — One experi- 
ment of Faraday deserves notice, as showing the part 
played by induction in these phenomena. He gradually 
lowered a charged metallic ball into a hollow conductor 
connected by a wire to a gold-leaf electroscope (Fig. 29), 
and watched the effect. A pewter ice-pail being con- 
venient for his purpose, this experiment is continually 
referred to by this name, though any other hollow con- 
ductor — a tin canister or a silver mug, placed on a 
glass support — would of course answer equally well. 
The following effects are observed : — Suppose the ball 
to have a -f charge : as it is lowered into the hollow con- 
ductor the gold leaves begin to diverge, for the presence 
of the charge acts inductively, and attracts a — charge 
into the interior and repels a -f charge to the exterior. 
The gold leaves diverge more and more until the ball 
is right within the hollow conductor, after which no 
greater divergence is obtained. On letting the ball 
touch the inside the gold leaves still remain diverging as 



DISTRIBUTION OF CHARGE 



43 






before, and if now the ball is pulled out it is found to 
have lost all its electrification. The fact that the gold 
leaves diverge no wider after the ball touched than they 
did just before, proves 
that when the charged 
ball is right inside the 
hollow conductor the 
induced charges are 
each of them precisely- 
equal in amount to its 
own charge, and the in- 
terior negative charge 
exactly neutralizes the 
charge on the ball at 
the moment when they 
touch, leaving the 
equal exterior charge 
unchanged. An electric 
cage, such as this ice- 
pail, when connected 
with an electroscope 
or electrometer, affords an excellent means of examining 
the charge on a body small enough to be hung inside 
it. For without using up any of the charge of the body 
(which we are obliged to do when applying the method 
of the proof-plane) we can examine the induced charge 
repelled to the outside of the cage, which is equal in 
amount and of the same sign. If two equal charges of 
opposite kinds are placed at the same time within the 
cage no effects are produced on the outside. 

38. Distribution of Charge. — A charge of electricity 
is not usually distributed uniformly over the surfaces 
of bodies. Experiment shows that there is more elec- 
tricity on the edges and corners of bodies than upon 
their flatter parts. This distribution can be deduced 
from the theory laid down in Lesson XXL, but mean- 
time we will give some of the chief cases as they can be 




Fig. 29. 



44 ELECTRICITY AND MAGNETISM part i 

shown to exist. The term Electric Density is used to 
signify the amount of electricity at any point of a sur- 
face; the electric density at a point is the number of units 
of electricity per unit of area (i.e. per square inch, or per 
square centimetre), the distribution being supposed uni- 
form over this small surface. 

(a) Sphere. — The distribution of a charge over an 
insulated sphere of conducting material is uniform, pro- 
vided the sphere is also isolated, that is to say, is remote 
from the presence of all other conductors and all other 
electrified bodies. The density is uniform all over it. 
This is symbolized by the dotted line round the sphere 




a h 

c 




<■_ d 



Fig. 30. 

in Fig. 30 a, which is at an equal distance from the 
sphere all round, suggesting an equal thickness oj: charge 
at every point of the surface. It must be remembered 
that the charge is not really of any perceptible thickness 
at all ; it resides on or at the surface, but cannot be said 
to form a stratum upon it. 

(b) Cylinder with rounded Ends. — Upon an elongated 
conductor, such as is frequently employed in electrical 
apparatus, the density is greatest at the ends where the 
curvature of the surface is the greatest. 

(c) Two Spheres in contact. — If two spheres in con- 
tact w r ith each other are insulated and charged, it is found 
that the density is greatest at the parts farthest from the 



chap, i DISTRIBUTION OF CHARGE 45 

point of contact, and least in the crevice between them. 
If the spheres are of unequal sizes the density is greater 
on the smaller sphere, which has the surface more curved. 
On an egg-shaped or pear-shaped conductor the density 
is greatest at the small end. On a cone the density is 
greatest at the apex ; and if the cone terminate in a 
sharp point the density there is very much greater than 
at any other point. At a point, indeed, the density of 
the collected electricity may be so great as to electrify 
the neighbouring particles of air, which then are repelled 
(see Art. 47), thus producing a continual loss of charge. 
For this reason points and sharp edges are always avoided 
on electrical apparatus, except where it is specially desired 
to set up a discharge. 

(7/) Flat Disk. — The density of a charge upon a flat 
disk is greater, as we should expect, at the edges than on 
the flat surfaces: bat over the flat surfaces the distribu- 
tion is fairly uniform. 

These various facts are ascertained by applying a 
small proof-plane successively at various points of the 
electrified bodies and examining the amount taken up by 
the proof-plane by means of an electroscope or electrome- 
ter. Coulomb, who investigated mathematically as well 
as experimentally many of the important cases of distri- 
bution, employed the torsion balance to verify his calcu- 
lations. He investigated thus the case of the ellipsoid of 
revolution, and found the densities of the charges at the 
extremities of the axis to be proportional to the lengths 
of those axes. He also -showed that the density of the 
charge at any other point of the surface of the ellipsoid 
was proportional to the length of the perpendicular drawn 
from the centre to the tangent at that point. Riess also 
investigated several interesting cases of distribution. He 
found the density at the middle of the edges of a cube to 
be nearly two and a half times as great as the density at 
the middle of a face ; while the density at a corner of the 
cube was more than four times as great. 



46 ELECTRICITY AXD MAGNETISM part i 

39. Redistribution of Charge. — If any portion of the 
charge of an insulated conductor be removed, the re- 
mainder of the charge will immediately redistribute itself 
over the surface in the same manner as the original 
charge, provided it be also isolated, i.e. that no other con- 
ductors or charged bodies be near to perturb the distri- 
bution by complicated effects of influence. 

If a conductor be charged with any quantity of elec- 
tricity, and another conductor of the same size and shape 
(but uncharged) be brought into contact with it for an 
instant and then separated, it will be found that the 
charge has divided itself equally between them. In the 
same way a charge may be divided equally into three 
or more parts by being distributed simultaneously over 
three or more equal and similar conductors brought into 
contact and symmetrically placed. 

If two equal metal balls, suspended by silk strings, 
charged with unequal quantities of electricity, are brought 
for an instant into contact and then separated, it will be 
found that the charge has redistributed itself fairly, half 
the sum of the two charges being now the charge of each. 
This may even be extended to the case of charges of 
opposite signs. Thus, suppose two similar conductors to 
be electrified, one with a positive charge of 5 units and 
the other with 3 units of negative charge, when these are 
made to touch and separated, each will have a positive 
charge of 1 unit ; for the algebraic sum of + 5 and — 3 is 
+ 2, which, shared between the two equal conductors, 
leaves + 1 for each. 

40. Capacity of Conductors. — If the conductors be 
unequal in size, or unlike in form, the shares taken by 
each in this redistribution will not be equal, but will be 
proportional to the electric capacities of the conductors. 
The definition of capacity in its relation to electric 
quantities is given in Lesson XXL, Art. 271. We may, 
however, make the remark, that two insulated conductors 
of the same form, but of different sizes, differ in their 



chap, i ELECTRIC MACHINES 47 

electrical capacity ; for the larger one must have a larger 
amount of electricity imparted to it in order to electrify 
its surface to the same degree. The term potential is 
employed in this connexion, in the following way : — A 
given quantity of electricity will electrify an isolated body 
up to a certain " potential " (or power of doing electric 
work) depending on its capacity. A large quantity of 
electricity imparted to a conductor of small capacity will 
electrify it up to a very high potential; just as a large 
quantity of water poured into a vessel of narrow capacity 
will raise the surface of the water to a high level in the 
vessel. The exact definition of Potential, in terms of 
energy spent against the electrical forces, is given in the 
lesson on Electrostatics (Art. 263). 

It will be found convenient to refer to a positively 
electrified body as one electrified to a positive or high 
potential: while a negatively electrified body may be 
looked upon as one electrified to a low or negative poten- 
tial. And just as we take the level of the sea as a zero 
level, and measure the heights of mountains above it, 
and the depths of mines below it, using the sea level as a 
convenient point of reference for differences of level, so 
we take the potential of the earth's surface (for the sur- 
face of the earth is always electrified to a certain degree) 
as zero potential, and use it as a convenient point of 
reference from which to measure differences of electric 
potential. 

Lesson Y. — Electric Machines 

41. For the purpose of procuring larger supplies of 
electricity than can be obtained by the rubbing of a rod 
of glass or shellac, electric machines have been devised. 
All electric machines consist of two parts, one for pro- 
ducing, the other for collecting, the electric charges. Ex- 
perience has shown that the quantities of + and — elec- 



48 ELECTRICITY AND MAGNETISM part i 

trification developed by friction upon the two surfaces 
rubbed against one another depend on the amount of 
friction, upon the extent of the surfaces rubbed, and also 
upon the nature of the substances used. If the two sub- 
stances employed are near together on the list of electrics 
given in Art. 6, the electrical effect of rubbing them 
together will not be so great as if two substances widely 
separated in the series are chosen. To obtain the highest 
effect, the most positive and the most negative of the 
substances convenient for the construction of a machine 
should be taken, and the greatest available surface of 
them should be subjected to friction, the moving parts 
having a sufficient pressure against one another compati- 
ble with the required velocity. 

The earliest form of electric machine was devised by 
Otto von Guericke of Magdeburg, and consisted of a 
globe of sulphur fixed upon a spindle, and pressed with 
the dry surface of the hands while being made to rotate; 
with this he discovered the existence of electric sparks 
and the repulsion of similarly electrified bodies. Sir 
Isaac Newton replaced Von Guericke's globe of sulphur 
by a globe of glass. A little later the form of the 
machine was improved by various German electricians; 
Von Bose added a collector or tk prime conductor," in the 
shape of an iron tube, supported by a person standing on 
cakes of resin to insulate him, or suspended by silken 
strings ; Winckler of Leipzig substituted a leathern 
cushion for the hand as a rubber; and Gordon of Erfurt 
rendered the machine more easy of construction by using 
a glass cylinder instead of a glass globe. The electricity 
was led from the excited cylinder or globe to the prime 
conductor by a metallic chain which hung over against 
the globe. A pointed collector was not employed until 
after Franklin's famous researches on the action of points. 
About 1760 De la Fond, Planta, Ramsden, and Cuthbert- 
son, constructed machines having glass plates instead of 
cylinders. All frictional machines are, however, now 



FRICTIONAL MACHINES 



49 



obsolete, having in recent years been quite superseded by 
the modern Influence Machines. 

42. The Cylinder Electric Machine. — The Cylinder 
Electric Machine consists of a glass cylinder mounted 
on a horizontal axis capable of being turned by a handle. 
Against it is pressed from behind a cushion of leather 
stuffed with horsehair, the surface of which is covered 
with a powdered amalgam of zinc or tin. A flap of silk 
attached to the cushion passes over the cylinder, covering 
its upper half. In front of the cylinder stands the 
"prime conductor," which is made of metal, and usually 




Fig. 31. 

of the form of an elongated cylinder with hemispherical 
ends, mounted upon a glass stand. At the end of the 
prime conductor nearest the cylinder is fixed a rod bear- 
ing a row of fine metallic spikes, resembling in form a 
rake ; the other end usually carries a rod terminated in 
a brass ball or knob. The general aspect of the machine 
is shown in Fig. 31. When the handle is turned the 
friction between the glass and the amalgam-coated sur- 
face of the rubber produces a copious electrical action, 
electricity appearing as a -f charge on the glass, leaving 
the rubber with a — charge. The prime conductor col- 



50 



ELECTRICITY AND MAGNETISM part i 



lects this charge by the following process : — The + charge 
being carried round on the glass acts inductively on the 
long insulated conductor, repelling a + charge to the far 
end ; leaving the nearer end — ly charged. The effect of 
the row of points is to emit a — ly electrified wind (see 
Art. 47) towards the attracting -f charge upon the glass, 
which is neutralized thereby ; the glass thus arriving 
at the rubber in a neutral condition ready to be again 
excited. This action of the points is sometimes described, 
though less correctly, by saying that the points collect the 
+ charge from the glass. If it is desired to collect also 
the — charge of the rubber, the cushion must be supported 
on an insulating stem and provided at the back with a 
metallic knob. It is, however, more usual to use only 
the + charge, and to connect the rubber by a chain to 
"earth," so allowing the — charge to be neutralized. 

43. The Plate Electric Machine. — The Plate Machine, 
as its name implies, is constructed with a circular plate 

of glass or of ebo- 
nite, and is usually 
provided with two 
pairs of rubbers 
formed of double 
cushions, pressing 
the plate between 
them, placed at its 
highest and lowest 
point, and provided 
with silk flaps, each 
H extending over a 
flj quadrant of the 
I circle. The prime 
conductor is either 
double or curved 
round to meet the 
plate at the two ends of its horizontal diameter, and is 
furnished with two sets of spikes, for the same purpose 




Fig. 32. 



chap, i USE OF FRICTIONAL MACHINES 51 

as the row of points in the cylinder machine. A common 
form of plate machine is shown in Fig. 32. The action 
of the machine is, in all points of theoretical interest, the 
same as that of the cylinder machine. Its advantages 
are that a large glass plate is more easy to construct than 
a large glass cylinder of perfect form, and that the length 
along the surface of the glass between the collecting row 
of points and the edge of the rubber cushions is greater 
in the plate than in the cylinder for the same amount of 
surface exposed to friction ; for, be it remarked, when the 
two charges thus separated have collected to a certain 
extent, a discharge will take place along this surface, the 
length of which limits therefore the power of the machine. 
In a more modern form, due to Le Roy, and modified by 
Winter, there is but one rubber and flap, occupying a 
little over a quadrant of the plate, and one collector or 
double row of points, while the prime conductor consists 
of a ring-shaped body. 

44. Electric Amalgam. — Canton, finding glass to be 
highly electrified when dipped into dry mercury, sug- 
gested the employment of an amalgam of tin with mercury 
as a suitable substance wherewith to cover the surface of 
the rubbers. Still better is Kienmayer's amalgam, con- 
sisting of equal parts of tin and zinc, mixed while molten 
with twice their weight of mercury. Bisulphide of tin 
("mosaic gold ") may also be used. These amalgams are 
applied to the cushions with a little stiff grease. They 
serve the double purpose of conducting away the negative 
charge separated upon the rnbber during the action of 
the machine, and of affording as a rubber a snbstance 
which is more powerfully negative (see list in Art. 6) than 
the leather or the silk of the cushion itself. Powdered 
graphite is also good. 

45. Precautions in using Factional Machines. — Sev- 
eral precautions must be observed in the use of elec- 
trical machines. Damp and dust must be scrupulously 
avoided. The surface of glass is hygroscopic, hence, 



52 ELECTKICITY AND MAGNETISM part i 

except in the driest climates, it is necessary to warm 
the glass surfaces and rubbers to dissipate the film of 
moisture which collects. Glass stems for insulation may 
be varnished with a thin coat of shellac varnish, or 
with paraffin (solid). A few drops of anhydrous paraffin 
(obtained by dropping a lump of sodium into a bottle of 
paraffin oil), applied with a bit of flannel to the pre- 
viously warmed surfaces, hinders the deposit of moist- 
ure. A frictional machine which has not been used for 
some months will require a fresh coat of amalgam on its 
rubbers. These should be cleaned and warmed, a thin 
uniform layer of tallow or other stiff grease is spread 
upon them, and the amalgam, previously reduced to a fine 
powder, is sifted over the surface. In spite of all pre- 
cautions friction machines are uncertain in their be- 
haviour in damp weather. This is the main reason why 
they have been superseded by influence machines, which 
do not need to be warmed. 

All points should be avoided in apparatus for frictional 
electricity except where they are desired, like the "col- 
lecting " spikes on the prime conductor, to let off a charge 
of electricity. All the rods, etc., in frictional apparatus 
are therefore made with rounded knobs. 

46. Experiments with the Electric Machine. — With 
the electric machine many pleasing and instructive ex- 
periments are possible. The phenomena of attraction and 
repulsion can be shown upon a large scale. Fig. 33 repre- 
sents a device knowm as the electric chimes,* in which 
two small brass balls hung by silk strings are set in 
motion and strike against the bells between wdiich they 
are hung. The two outer bells are hung by metallic 
wires or chains to the knob of the machine. The third 
bell is hung by a silk thread, but communicates with the 
ground by a brass chain. The balls are first attracted to 

* Invented in 1752 by Franklin, for the purpose of warning him of the 
presence of atmospheric electricity, drawn from the air above his house by 
a pointed iron rod. 



chap, i EXPERIMENTS WITH MACHINES 



53 



the electrified outer bells, then repelled, and, having dis- 
charged themselves against the uninsulated central bell, 
are again attracted, and so vibrate to and fro. 

By another arrangement small figures or dolls cut out 
of pith can be made to dance up and down between a 
metal plate hung horizontally 
from the knob of the machine, 
and another flat plate an inch 
or two lower and communi- 
cating with "earth." 

Another favourite way of 
exhibiting electric repulsion 
is by means of a doll with 
long hair placed on the ma- 
chine ; the individual hairs 
stand on end when the ma- 
chine is worked, being re- 
pelled from the head, and 
from one another. A paper 
tassel will behave similarly 
if hung to the prime con- 
ductor. The most striking way of showing this pheno- 
menon is to place a person upon a gla-s-legged stool, 
making him touch the knob of the machine ; when the 
machine is worked, his hair, if dry, will stand on end. 
Sparks will pass freely between a person thus electrified 
and one standing upon the ground. 

The sparks from the machine may be made to kindle 
spirits of wine or ether, placed in a metallic spoon, con- 
nected by a wire, with the nearest metallic conductor 
that runs into the ground. A gas jet may be lit by 
passing a spark to the burner from the finger of the 
person standing, as just described, upon an insulating 
stool. 

47. Effect of Points ; Electric Wind. — The effect of 
points in discharging electricity from the surface of a con- 
ductor may be readily proved by numerous experiments. 




Fig. 33. 



54 



ELECTRICITY AND MAGNETISM tart 



If the machine be in good working order, and capable of 
giving, say, sparks 4 inches long when the knuckle is 
presented to the knob, it will be found that, on fastening 
a fine pointed needle to the conductor, it discharges the 
electricity so effectually at its point that only the shortest 
sparks can be drawn at the knob, while a fine jet or brush 
of pale blue light will appear at the point. If a lighted 
taper be held in front of the point, the flame will be 
visibly blown aside (Fig. 34) by the streams of electrified 
air repelled from the point. These air-currents can be 




Fiff. 34. 



felt with the hand. They are due to a mutual repulsion 
between the electrified air particles near the point and 
the electricity collected on the point itself. That this 
mutual reaction exists is proved by the electric fly or 
electric reaction-mill of Hamilton (Fig. 35), which con- 
sists of a light cross of brass or straw, suspended on a 
pivot, and having the pointed ends bent round at right 
angles. When placed on the prime conductor of the 
machine, or joined to it by a chain, the force of repulsion 
between the electricity of the points and that on the air 



chap, i ELECTRIC WINDS FROM POINTS 



55 





immediately in front of them drives the mill round in 
the direction opposite to that in which the points are 
bent. It will even rotate if immersed in turpentine or 
petroleum. If the points of the 
fly are covered with small round 
lumps of wax it will not rotate, 
as the presence of the wax pre- 
vents the formation of any 
wind or stream of electrified 
particles. 

The electric wind from a 
point will produce a charge 
upon the surface of any insulat- 
ing body, such as a plate of 
ebonite or glass, held a few 
inches away. The charge may 
be examined by dusting red 
lead or lycopodium powder 
upon the surface. If a slip of 
glass or mica be interposed between the point and the 
surface against which the wind is directed, an electric 
shadow will be formed on the surface at the part so 
screened. 

48. Armstrong's Hydro-Electrical Machine. — The 
friction of a jet of steam issuing from a boiler, through 
a wooden nozzle, generates electricity. In reality it is 
the particles of condensed water in the jet which are 
directly concerned. Sir W. Armstrong, who investigated 
this source of electricity, constructed a powerful appara- 
tus, known as the hydro-electrical machine, capable of 
producing enormous quantities of electricity, and yield- 
ing sparks 5 or 6 feet long. The collector consisted of 
a row of spikes, placed in the path of the steam jets 
issuing from wooden nozzles, and was supported, together 
with a brass ball which served as prime conductor, upon 
a glass pillar. 

49. Influence Machines. — There is another class of 



Fig. 35. 



56 ELECTRICITY AND MAGNETISM part i 

electrical machine, differing entirely from those we have 
been describing, and depending upon the principle of 
influence. They also have been termed concection-ind ac- 
tion machines, because they depend upon the employment 
of a minute initial charge which, acting by influence, 
induces other charges, which are then conveyed by the 
moving parts of the machine to some other part, where 
they can be used either to increase the initial charge or to 
furnish a supply of electrification to a suitable collector. 
Of such instruments the oldest is the Electrophorus, ex- 
plained fully in Lesson III. Bennet, Nicholson, Erasmus 
Darwin, and others devised pieces of apparatus for ac- 
complishing by mechanism that which the electrophorus 
accomplishes by hand. Nicholson's revolving doubter, in- 
vented in 1788, consists of a revolving apparatus, in which 
an insulated carrier can be brought into the presence of an 
electrified body, there touched for an instant while under 
influence, then carried forward with its acquired charge 
towards another body, to which it imparts its charge, and 
which in turn acts inductively on it, giving it an opposite 
charge, which it can convey to the first body, thus 
increasing its initial charge at every rotation. 

In the modern influence machines two principles are 
embodied: (1) the principle of influence, namely, that a 
conductor touched while under influence acquires a charge 
of the opposite kind; (2) the principle of reciprocal accu- 
mulation. This principle must be carefully noted. Let 
there be two insulated conductors A and B electrified ever 
so little, one positively, the other negatively. Let a third 
insulated conductor C, which will be called a carrier, be 
arranged to move so that it first approaches A and then 
B, and so forth. If touched while under the influence 
of the small positive charge on A it will acquire a small 
negative charge ; suppose that it then moves on and 
gives this negative charge to B. Then let it be touched 
while under the influence of B, so acquiring a small 
positive charge. When it returns towards A let it give 



CHAP. I 



INFLUENCE MACHINES 



57 



up this positive charge to A, thereby increasing its 
positive charge. Then A will act more powerfully, and 
on repeating the former operations both B and A will 
become more highly charged. Each accumulates the 
charges derived by influence from the other. This is the 
fundamental action of the machines in question. The 
modern influence machines date from I860, when C. F. 
Varley produced a form with six carriers mounted on a 
rotating disk of glass. This was followed in 1805 by 




Fig. 36. 

the machine- of Holtz and that of Toepler, and in 1867 
by those of Lord Kelvin (the " replenisher " and the 
" mouse-mill "). The latest forms are those of Mr. 
James Wimshurst. 

50. Typical Construction. — Before describing some 
special forms we will deal with a generalized type of 
machine having two fixed Jield-plates, A and B, which 
are to become respectively + and — , and a set of carriers, 
attached to a rotating disk or armature. Fig. 36 gives in 



58 ELECTRICITY AND MAGNETISM part i 

a diagrammatic way a view of the essential parts. For 
convenience of drawing it is shown as if the metal field- 
plates A and B were affixed to the outside of an outer 
stationary cylinder of glass ; the six carriers p, q, ?*, s, t, 
and u being attached to the inside of an inner rotating 
cylinder. The essential parts then are as follows : — 

(i.) A pair of ji eld-plates A and B. 

(ii.) A set of rotating carriers p, q, r, s, t, and u. 

(iii.) A pair of neutralizing brushes n v n 2 made of 
flexible metal wires, the function of which is 
to touch the carriers while they are under the 
influence of the field-plates. They are con- 
nected together by a diagonal conductor, which 
need not be insulated. 

(iv.) A pair of appropriating brushes a v </.„ which reach 
over from the field-plates to appropriate the 
charges that are conveyed around by the 
carriers, and impart them to the field-plates. 

(v.) In addition to the above, which are sufficient to 
constitute a complete self-exciting machine, it 
is usual to add a discharging apparatus, con- 
sisting of two combs c v c 2 , to collect any unap- 
propriated charges from the carriers after they 
have passed the appropriating brushes; these 
combs being connected to the adjustable dis- 
charging balls at D. 

The operation of the machine is as follows. The 
neutralizing brushes are set so as to touch the moving 
carriers just before they pass out of the influence of the 
field-plates. Suppose the field-plate A to be charged ever 
so little positively, then the carrier /), touched by ??j just 
as it passes, will acquire a slight negative charge, which 
it will convey forward to the appropriating brush a v and 
will thus make B slightly negative. Each of the carriers 
as it passes to the right over the top will do the same 
thing. Similarly each of the carriers as it passes from 



chap, i INFLUENCE MACHINES 59 

right to left at the lower side will be touched by n 2 while 
under the influence of the — charge on B, and will 
convey a small -f charge to A through the appropriating 
brush a 2 . In this way A will rapidly become more and 
more -f , and B more and more — ; and the more highly 
charged they become, the more do the collecting combs 
c x and c 2 receive of unappropriated charges. Sparks will 
snap across between the discharging knobs at D. 

The machine will not be self-exciting unless there is a 
good metallic contact made by the neutralizing brushes and 
by the appropriating brushes. If the discharging apparatus 
were fitted at c v c 2 with contact brushes instead of spiked 
combs, the machine would be liable to lose the charge of 
the field-plates, or even to have their charges reversed in 
sign whenever a large spark was taken from the knobs. 

It will be noticed that there are two thicknesses of 
glass between the fixed field- platen and the rotating carriers. 
The glass serves not only to hold the metal parts, but 
prevents the possibility of back-discharges (by sparks or 
winds) from the carriers to the field-plates as they pass. 

The essential features thus set forth will be found in 
Var ley's machine of 1860, in Lord Kelvin's "replenisher" 
(which had only two carriers), and in many other machines 
including the apparatus known as Clarke's "gas-lighter." 

51. Toepler's Influence Machine. — In this machine, 
as constructed by Voss, are embodied various points due 
to Holtz and others. Its construction follows almost 
literally the diagram already explained, but instead of 
having two cylinders, one inside the other, it has two 
flat disks of varnished glass, one fixed, the other slightly 
smaller rotating in front of it (Fig. 37). The field-plates 
A and B consist of pieces of tinfoil, cemented on the 
back of the back disk, each protected by a coating of 
varnished paper. The carriers are small disks or sectors 
of tinfoil, to the number of six or eight, cemented to the 
front of the front disk. To prevent them from being 
worn away by rubbing against the brushes a small 



60 



ELECTKICITY AND MAGNETISM part i 



metallic button is attached to the middle of each. The 
neutralizing brushes n v n 2 are small whispa of fine 
springy brass wire, and are mounted on the ends of a 
diagonal conductor Z. The appropriating brushes <7 r a 2 
are also of thin brass wire, and are fastened to clamps 
projecting from the edge of the fixed disk, so that they 
communicate metallically with the two field-plates. The 
collecting combs, which have brass spikes so short as not 
to touch the carriers, are mounted on insulating pillars 
and are connected to the adjustable discharging knobs 




BACK FIXED DISK WITH 
FIELD PLATES ON BACK. 



FRONT ROTATING DISK 
WITH CARRIERS ON FRONT. 



Fig. 37. 



D r D 2 . These also communicate with two small Leyden 
jars J v J 2 , the function of which is to accumulate the 
charges before any discharge takes place. These jars are 
separately depicted in Fig. 38. Without them, the dis- 
charges between the knobs take place in frequent thin 
blue sparks. With them the sparks are less numerous, 
but very brilliant and noisy. 

To use the Toepler (Voss) machine first see that all 
the four brushes are so set as to make good metallic con- 
tact with the carriers as they move past, and that the 



chap, i TOEPLER (VOSS) INFLUENCE MACHINE 01 

neutralizing brushes are set so as to touch the carriers 
while under influence. Then see that the discharging 
knobs are drawn widely apart. Set the machine in 
rotation briskly. If it is clean it should excite itself 
after a couple of turns, and will emit a gentle hissing 
sound, due to internal discharges (visible as blue glimmers 
in the dark), aud will offer more resistance to turning. 
If then the knobs are pushed nearer together sparks will 
pass across between them. The jars (the addition of 
which we owe to Holtz) should be kept free from dust. 
Sometimes a pair of terminal screws are added at S r S 9 
(Fig. 38), connected respectively with the outer coatings 







Fig. 35. 



of the jars. These are convenient for attaching wires to 
lead away discharges for experiments at a distance. If 
not so used they should be joined together by a short 
wire, as the two jars will not work properly unless their 
outer coatings are connected. 

52. Wimshurst's Influence Machine. — In this, the 
most widely used of influence machines, there are no 
fixed field-plates. In its simplest form it consists (Fig. 
39) of two circular plates of varnished glass, which are 
geared to rotate in opposite directions. A number of 
sectors of metal foil are cemented to the front of the 
front plate and to the back of the back plate; these 
sectors serve both as carriers and as inductors. Across 



62 



ELECTRICITY AND MAGNETISM part i 



the front is fixed an uninsulated diagonal conductor, 
carrying at its ends neutralizing brushes, which touch 
the front sectors as they pass. Across the back, but 
sloping the other way, is a second diagonal conductor, 
with brushes that touch the sectors on the hinder plate. 
Nothing more than this is needed for the machine to 
excite itself when set in rotation ; but for convenience 




there is added a collecting and discharging apparatus. 
This consists of two pairs of insulated combs, each pair 
having its spikes turned inwards toward the revolving 
disks, but not touching them; one pair being on the 
right, the other on the left, mounted each on an insulat- 
ing pillar of ebonite. These collectors are furnished 
with a pair of adjustable discharging knobs overhead; 



chap, i WIMSHURST INFLUENCE MACHINE 



63 



and sometimes a pair of Leyden jars is added, to prevent 
the sparks from passing until considerable quantities of 
charge have been collected. 

The processes that occur in this machine are best 
explained by aid of a diagram (Fig. 40), in which, for 
greater clearness, the two rotating plates are represented 




Fig. 40. 



as though they were two cylinders of glass, rotating 
opposite ways, one inside the other. The inner cylinder 
will represent the front plate, the outer the back plate. 
In Figs. 39 and 40 the front plate rotates right-handedly, 
the back plate left-handedly. The neutralizing brushes 
n v n 2 touch the front sectors, while n 3 , ?i± touch against 
the back sectors. 



64 ELECTRICITY AND MAGNETISM part i 

Now suppose any one of the back sectors represented 
near the top of the diagram to receive a slight positive 
charge. As it is moved onward toward the left it will 
come opposite the place where one of the front sectors is 
moving past the brush n v The result will be that the 
sector so touched while under influence by n 1 will acquire 
a slight negative charge, which it will carry onwards 
toward the right. When this negatively-charged front 
sector arrives at a point opposite n 3 it acts inductively on 
the back sector which is being touched by n 3 ; hence 
this back sector will in turn acquire a positive charge, 
which it will carry over to the left. In this way all the 
sectors will become more" and more highly charged, the 
front sectors carrying over negative charges from left to 
right, and the back sectors carrying over positive charges 
from right to left. At the lower half of the diagram a 
similar but inverse set of operations will be taking place. 
For when n } touches a front sector under the influence of 
a positive back sector, a repelled charge will travel along 
the diagonal conductor to n v helping to charge positively 
the sector which it touches. The front sectors, as they 
pass from right to left (in the lower half), will carry 
positive charges, while the back sectors, after touching 
n 4 , will carry negative charges from left to right. The 
metal sectors then act both as carriers and as inductors. 
It is clear that there will be a continual carrying of posi- 
tive charges toward the right, and of negative charges 
to the left. At these points, toward which the opposite 
kinds of charges travel, are placed the collecting-combs 
communicating with the discharging knobs. The latter 
ought to be opened wide apart when starting the machine, 
and moved together after it has excited itself. 

In larger Wimshurst influence machines two, three, 
or more pairs of oppositely-rotating plates are mounted 
within a glass case to keep off the dust. If the neutral- 
izing brushes make good metallic contact these machines 
are all self-exciting in all weathers. Machines with only 



CHAP. I 



HOLTZ INFLUENCE MACHINE 



65 



six or eight sectors on each plate give longer sparks, but 
less frequently than those that have a greater number. 
Mr. Wimshurst has designed many influence machines, 
from small ones with disks 2 inches across up to that at 
South Kensington, which has plates 7 feet in diameter. 

Prior to Wimshurst's machine Holtz had constructed 
one with two oppositely-rotating glass disks ; but they 
had no metal carriers upon them. It was not self-exciting. 

53. Holtz's Influence Machine. — The Holtz machine 
in its typical form had the following peculiarities. 
There were no metal carriers upon the rotating plate, 
hence another mode of charging it had to be adopted in 
lieu of touching conductors 
while under influence, 
as will be seen. The 
field-plates A and B (Fig. 
41) were of varnished 
paper — a poor conductor 
— fastened upon the back 
of the fixed disk. In the 
fixed disk of glass, on 
which the field-plates were 
mounted, there were cut 
two windows or openings, 
through which there pro- 
jected from the field-plates two pointed paper tongues, 
which took the place of appropriating brushes. The 
discharging knobs were inserted in the neutralizing cir- 
cuit which united two metal combs with pointed spikes, 
situated in front of the rotating front disk, opposite the 
two field-plates. There was (at first) no diagonal con- 
ductor. It will be noted that while the combs, which 
served both as neutralizing and collecting combs, were in 
front of the rotating plate, the appropriating tongues 
were situated at the back of the same. Fig. 41 is a 
view of the machine from behind. The machine was 
not self-exciting. In operating it the following procedure 




Fig. 41. 



66 ELECTRICITY AND MAGNETISM part i 

was used : first the two discharging knobs were put 
together, then the front disk was set into rapid rotation. 
While so rotating a small initial charge was communi- 
cated to one of the field-plates by holding to it a rubbed 
piece of ebonite or glass, or by sending into it a spark 
from a Leyden jar. Thereupon the machine charged 
itself, and began to emit pale blue sparks from the points 
of the combs and tongues with a hissing sound. On then 
drawing apart the discharging knobs, a torrent of sparks 
rushed across. 

These arrangements being known, it is not difficult 
to follow the action of the machine, provided it is once 
understood that the whole operation depends upon the 
circumstance that the surface of a non-conducting body 
such as glass can be electrified by letting off against it 
an electric w T ind from a point placed near it (see Art. 47). 
Suppose that a small initial + charge is given to A. This 
will operate by influence upon the metal parts imme- 
diately opposite it, and cause the spikes to become elec- 
trified negatively, and to give off a negatively electrified 
wind, which will charge the face of the rotating plate, 
these charges being then carried over to the other side, 
where the spikes of the other comb will be emitting a 
positively electrified wind. The pointed tongues which 
project towards the back of the rotating disk also let off 
w T inds, the tendency being always for them to charge the 
back of the plate with a charge of opposite sign from 
that which is coming toward them on the front. If 
negative charges are being carried over the top on the 
front, then the tongue of B will tend to let off a positive 
charge against the back, thereby leaving B more negative. 
In the same way the tongue of A will let off a negatively 
electrified wind, making A more positive, so building up 
or accumulating two opposite kinds of charges on the 
two field-plates. This action will not occur unless the 
moving plate rotates in the direction opposite to that in 
which the two tongues point. 



chap, i HOLTZ INFLUENCE MACHINE 67 

• The defects of the Holtz machine were that it was so 
sensitive to damp weather as to be unreliable, that it was 
apt suddenly to reverse its charges, and that the electric 
winds by which it operated could not be produced with- 
out a sufficiently great initial charge. 

In later Holtz machines a number of rotating disks 
fixed upon one common axis were employed, the whole 
being enclosed in a glass case to prevent the access of 
damp. A small disk of ebonite was sometimes fixed to 
the same axis, and provided with a rubber, in order to 
keep up the initial charge by friction. Holtz constructed 
many forms of machine, including one with thirty-two 
plates, besides machines of a second kind having two 
glass plates rotating in opposite directions. 

The Holtz machine, as indeed every kind of influence 
machine, is reversible in its action ; that is to say, that if 
a continuous supply of the two electricities (furnished by 
another machine) be communicated to the armatures, the 
movable plate will be thereby set in rotation and, if 
allowed to run quite freely, will turn in an opposite sense. 

Righi showed that a Holtz machine can yield a con- 
tinuous current like a voltaic battery, the strength of 
the current being nearly proportional to the velocity of 
rotation. It was found that the electromotive-force of a 
machine was equal to that of 52,000 Darnell's cells, or 
nearly 53,000 volts, at all speeds. The resistance when 
the machine made 120 revolutions per minute was 2810 
million ohms; but only 646 million ohms when making 
450 revolutions per minute. 

54. Experiments with Influence Machines. — The 
experiments described in Art. 43, and indeed all those 
usually made with the old frictional machines, includ- 
ing the charging of Ley den jars, can be performed 
by the aid of influence machines. In some cases it is 
well to connect one of the two discharging knobs to the 
earth by a wire or chain, and to take the discharge from 
the other knob. To illuminate small vacuum tubes they 



68 ELECTRICITY AND MAGNETISM part i 

may be connected by guttapercha-covered wires to the 
two discharging knobs, or to the terminals S r S 2 of 
Fig. 38. The curious property of the electric discharge 
from a point in collecting dust or fumes is readily shown 
by connecting by a wire a needle which is introduced 
into a bell-jar of glass. The latter is filled with fumes 
by burning inside it a bit of magnesium wire, or of brown 
paper. Then on turning the handle of the influence 
machine the fumes are at once deposited, and the air left 
clear. 



Lesson VI. — The Leyden Jar and other Condensers 

55. It was shown in previous lessons that the opposite 
charges of electricity attract one another; that electricity 
cannot flow through glass; and that yet electricity can 
act across glass by influence. Two suspended pith-balls, one 
electrified positively and the other negatively, will attract 
one another across the intervening air. If a plate of glass 
be put between them they will still attract one another, 
though neither they themselves nor the electric charges 
on them can pass through the glass. If a pith-ball 
electrified with a — charge be hung inside a dry glass 
bottle, and a rubbed glass rod be held outside, the pith- 
ball will rush to the side of the bottle nearest to the glass 
rod, being attracted by the -f charge thus brought near it. 
If a pane of glass be taken, and a piece of tinfoil be stuck 
upon the middle of each face of the pane, and one piece 
of tinfoil be charged positively, and the other negatively, 
the two charges will attract one another across the glass, 
and will no longer be found to be free. If the pane is 
set up on edge, so that neither piece of tinfoil touches the 
table, it will be found that hardly any electricity can be 
got by merely touching either of the foils, for the charges 
are " bound," so to speak, by each other's attractions ; 
each charge is inducing the other. In fact it will be 



CONDENSERS 



69 



found that these two pieces of tinfoil may be, in this 
manner, charged a great deal more strongly than either of 
them could possibly be if it were stuck to a piece of glass 
alone, and then electrified. In other words, the capacity 
of a conductor is greatly increased when it is placed near to a 
conductor electrified with the opposite kind of charge. If its 
capacity is increased, a greater quantity of electricity may 
be put into it before it is charged to an equal degree of 
potential. Hence, such an arrangement for holding a 
large quantity of electrification may be called a condenser 
of electricity. 

56. Condensers. — Next, suppose that we have two 
brass disks, A and B (Fig. 42), set upon insulating stems, 
and that a glass plate is placed between them. Let B be 
connected by a wire 
to the knob of an C 

electrical machine, 
and let A be joined 
by a wire to "earth." 
The + charge upon 
B will act induc- 
tively across the 
glass plate on A, 
and will repel elec- Fig. 42. 

tricity into the earth, 

leaving the nearest face of A negatively electrified. This 
— charge on A will attract the + charge of B to the side 
nearest the glass, and a fresh supply of electricity will come 
from the machine. Thus this arrangement will become a 
condenser. If the two brass disks are pushed up close to 
the glass plate there will be a still stronger attraction 
between the -f- and — charges, because they are now nearer 
one another, and the inductive action will be greater; hence 
a still larger quantity can be accumulated in the plates. 
We see then that the capacity of a condenser is increased 
by bringing the plates near together. If now, while the 
disks are strongly charged, the wires are removed and the 





70 ELECTKICITY AND MAGNETISM part i 

disks are drawn backwards from one another, the two 
charges will not hold one another bound so strongly, and 
there will be more free electrification than before over their 
surfaces. This would be rendered evident to the experi- 
menter by the little pith-ball electroscopes fixed to them 
(see the Fig.), which would rly out as the brass disks were 
moved apart. We have put no further charge on the 
disk B, and yet, from the indications of the electroscope, 
we should conclude that by moving it away from disk A 
it has become electrified to a higher degree. The fact is, 
that while the conductor B was near the — charge of A 
the capacity of B was greatly increased, but on moving it 
away from A its capacity has diminished, and hence the 
same quantity of electricity now electrifies it to a higher 
degree than before. The presence, therefore, of an earth- 
connected plate near an insulated conductor increases its 
capacity, and permits it to accumulate a greater charge 
by attracting and condensing the electricity upon the face 
nearest the earth-plate, the surface-density on this face 
being therefore very great ; hence the appropriateness of 
the term condenser as applied to the arrangement. It was 
formerly also called an accumulator ; but the term accu- 
mulator is now reserved for the special kind of battery for 
storing the energy of electric currents (Art. 492). 

The stratum of air between the two disks will suffice 
to insulate the two charges one from the other. The 
brass disks thus separated by a stratum of air constitute 
an air-condenser, or air-leyden. Such condensers were 
first devised by Wilcke and Aepinus. In these experi- 
ments the sheet of glass or layer of air acts as a dielectric 
(Art. 295) conveying the inductive action through its 
substance. All dielectrics are insulators, but equally 
good insulators are not necessarily equally good dielec- 
trics. Air and glass are far better insulators than ebonite 
or paraffin in the sense of being much worse conductors. 
But influence acts more strongly across a slab of glass 
than across a slab of ebonite or paraffin of equal thickness, 



DISPLACEMENT 71 



and better still across these than across a layer of air. In 
other words, glass is a better dielectric than ebonite, or 
paraffin, or air, as it possesses a higher inductive capacity. 
It will then be seen that in the act of charging a con- 
denser, as much electricity flows out at one side as flows 
in at the other. 

57. Displacement. — Whenever electric forces act on 
a dielectric, tending to drive electricity in at one side and 
out at the other, we may draw lines of force through the 
dielectric in the direction of the action, and we may con- 
sider tubular spaces mapped out by such lines. We may 
consider a tube of electric force having at one end a 
definite area of the positively charged surface, and at the 
other end an area of the negatively charged surface. 
These areas may be of different size or shape, but the 
quantities of -f and — electrification over them will be 
equal. The quantity of electricity which has apparently 
been transferred along the tube was called by Maxwell 
"the displacement" In non-conductors it is proportional 
to the electromotive-force. In conductors electromotive 
forces produce currents, which may be regarded as dis- 
placements which increase continuously with time. In 
certain crystalline media the displacement does not take 
place exactly in the direction of the electric force : in 
this case we should speak of tubes of influence rather 
than tubes of force. A unit tube will be bounded at its 
two ends by unit charges + and — . We may consider 
the whole electric field between positively and negatively 
charged bodies as mapped out into such tubes. 

58. Capacity of a Condenser. — It appears, therefore, 
that the capacity of a condenser will depend upon — 

(1) The size and form of the metal plates or coatings. 

(2) The thinness of the stratum of dielectric between 

them; and 

(3) The dielectric capacity of the material. 

59. The Leyden Jar. — The Leyden jar, called after 
the city where it was invented, is a convenient form of 




72 ELECTRICITY AND MAGNETISM part i 

condenser. It usually consists (Fig. 43) of a glass jar 
coated up to a certain height on the inside and outside 
with tinfoil. A brass knob fixed on the end of a stout 
brass wire passes downward through a lid or top of dry 
well-varnished wood, and communicates by a loose bit of 
brass chain with the inner coating of foil. To charge the 
jar the knob is held to the prime conductor of an electrical 

machine, the outer coating 
being either held in the hand 
or connected to " earth " by a 
wire or chain. When a + 
charge of electricity is im- 
parted thus to the inner coat- 
ing, it acts inductively on the 
outer coating, attracting a — 
charge into the face of the 
outer coating nearest the glass, 
and repelling a -f charge to the 
outside of the outer coating, 
and thence through the hand or wire to earth. After 
a few moments the jar will have acquired its full 
charge, the outer coating being — and the inner -f. If 
the jar is of good glass, and dry, and free from dust, it 
will retain its charge for many hours or days. But if a 
path be provided by which the two mutually attracting 
electricities can flow to one another, they will do so, and 
the jar will be instantaneously discharged. If the outer 
coating be grasped with one hand, and the knuckle of the 
other hand be presented to the knob of the jar, a bright 
spark will pass between the knob and the knuckle with 
a sharp report, and at the same moment a convulsive 
" shock " will be communicated to the muscles of the 
wrists, elbows, and shoulders. A safer means of dis- 
charging the jar is afforded by the discharging tongs 
or discharger (Fig. 44), which consists of a jointed brass 
rod provided with brass knobs and a glass handle. One 
knob is laid against the outer coating, the other is then 



LEYDEN JAR 73 




brought near the knob of the jar, and a bright snapping 

spark leaping from knob to knob announces that the two 

accumulated charges have flowed 

together, completing the discharge. 

Sometimes a jar discharges itself by 

a spark climbing over the top edge of 

the jar. Often when a jar is well 

charged a hissing sound is heard, due 

to partial discharges creeping over 

the edge. They can be seen in the 

dark as pale phosphorescent streams. 

60. Discovery of the Ley den Jar. 
— The discovery of the Leyden jar 
arose from the attempt of Musschen- 
broek and his pupil Cuneus * to col- 
lect the supposed electric " fluid " in a bottle half filled 
with water, which was held in the hand and was provided 
with a nail to lead the " fluid " down through the cork 
to the water from the electric machine. Here the water 
served as an inner coating and the hand as an outer 
coating to the jar. Cuneus on touching the nail received 
a shock. This accidental discovery created the greatest 
excitement in Europe and America. 

61. Residual Charges. — If a Leyden jar be charged 
and discharged and then left for a little time to itself, 
it w r ill be found on again discharging that a small 
second spark can be obtained. There is in fact a 
residual charge which seems to have soaked into the 
glass or been absorbed. The return of the residual 
charge is hastened by tapping the jar. The amount of 
the residual charge varies with the time that the jar has 
been left charged; it also depends on the kind of the 
glass of which the jar is made. There is no residual 
charge discoverable in an air-ley den after it has once 
been discharged. 

* The honour of the invention of the jar is also claimed for Kleist, 
Bishop of Pomerania. 



74 



ELECTRICITY AND MAGNETISM part i 



62. Batteries of Leyden Jars. — A large Leyden jar 
will give a more powerful shock than a small one, for a 
larger charge can be put into it ; its capacity is greater. 
A Leyden jar made of thin glass has a greater capacity 
as a condenser than a thick one of the same size; but if 
it is too thin it will be destroyed when powerfully charged 




Fig. 45. 

by a spark actually piercing the glass. " Toughened " 
glass is less easily pierced than ordinary glass, and hence 
Leyden jars made of it may be made thinner, and so will 
hold a greater charge. To prevent jars from being pierced 
by a spark, the highest part of the inside coating should 
be connected across by a strip of foil or a metallic disk 
to the central wire. 

If a jar is desired to give long sparks, there must be 



CHAP. I 



LEYDEN JARS 



75 



left a long space of varnished glass above the top of the 
coatings. 

If it is desired to accumulate a very great charge of 
electricity, a number of jars must be employed, all their 
inner coatings being connected together, and all their 
outer coatings being united. This arrangement is called 
a battery of Leyden jars, or Leyden battery (Fig. 45). 
As it has a large capacity, it will require a large quantity 
of electricity to charge it fully. When charged it pro- 
duces very powerful effects ; its spark will pierce glass 
readily, and every care must be 
taken to avoid a shock from it 
passing through the person, as it 
might be fatal. The " Universal 
Discharger " as employed with the 
Leyden battery is shown at the 
right of the figure. 

63. Seat of the Charge. — Ben- 
jamin Franklin discovered that the 
charges of the Leyden jar really 
reside on the surface of the glass, 
not on the metallic coatings. This 
he proved by means of a jar whose 
coatings could be removed (Fig. 
46). The jar was charged and 
placed upon an insulating stand. 
The inner coating was then lifted 
out, and the glass jar was then 
taken out of the outer coating. 
Neither coating was found to be 
electrified to any extent, but on 
again putting the jar together it 
was found to be highly charged. The charges had all the 
time remained upon the inner and outer surfaces of the 
glass dielectric. 

64. Dielectric Strain. — Farady proved that the me- 
dium across which influence takes place really plays an 




Fig. 46. 



76 ELECTRICITY AND MAGNETISM part i 

important part in the phenomena. It is now known 
that all dielectrics across which inductive actions are at 
work are thereby strained.* Inasmuch as a good vacuum 
is a good dielectric, it is clear that it is not necessarily 
the material particles of the dielectric substance that are 
thus affected; hence it is believed that electrical pheno- 
mena are due to stresses and strains in the so-called 
" ether," the thin medium pervading all matter and all 
space, whose highly elastic constitution enables it to con- 
vey to us the vibrations of light though it is millions of 
times less dense than air. As the particles of bodies are 
intimately surrounded by ether, the strains of the ether 
are also communicated to the particles of bodies, and they 
too suffer a strain. The glass between the two coatings 
of tinfoil in the Leyden jar is actually strained or 
squeezed, there being a tension along the lines of electric 
force. When an insulated charged ball is hung up in a 
room an equal amount of the opposite kind of charge is 
attracted to the inside of the walls, and the air between 
the ball and the walls is strained (electrically) like the 
glass of the Leyden jar. If a Leyden jar is made of thin 
glass it may give way under the stress; and when a 
Leyden jar is discharged the layer of air between the 
knob of the jar and the knob of the discharging tongs is 
more and more strained as they are approached towards 
one another, till at last the stress becomes too great, and 
the layer of air gives way, and is " perforated " by the 
spark that discharges itself across. The existence of such 
stresses enables us to understand the residual charge of 
Leyden jars in which the glass does not recover itself all 
at once, by reason of its viscosity, from the strain to 
which it has been subjected. It must never be for- 
gotten that electric force acts across space in conse- 
quence of the transmission of stresses and strains in the 

* In the exact sciences a strain means an alteration of form or volume 
due to the application of a stress. A stress is the force, pressure, or other 
agency which produces a strain. 



chap, i OTHER SOURCES 77 

medium with which space is filled. In every case we 
store not electricity but energy. Work is done in push- 
ing electricity from one place to another against the 
forces which tend to oppose the movement. The charg- 
ing of a Leyden jar may be likened to the operation of 
bending a spring, or to pumping up water from a low 
level to a high one. In charging a jar we pump exactly 
as much electricity out of the negative side as we pump 
into the positive side, and we spend energy in so doing. 
It is this stored energy which afterwards reappears in 
the discharge. 



Lesson VII. — Other Sources of Electrification 

G5. It was remarked at the close of Lesson I. 
(p. 13) that friction was by no means the only source 
of electricity. Some of the other sources will now be 
named. 

G6. Percussion. — A violent blow struck by one sub- 
stance upon another produces opposite electrical states 
on the two surfaces. It is possible indeed to draw up a 
list resembling that of Art. 6, in such an order that each 
substance will take a -f charge on being struck with one 
lower on the list. 

67. Vibration. — Volpicelli showed that vibrations 
set up within a rod of metal coated with sulphur or 
other insulating substance, produced a separation of 
electricities at the surface separating the metal from the 
non-conductor. 

68. Disruption and Cleavage. — If a card be torn 
asunder in the dark, sparks are seen, and the separated 
portions, when tested with an electroscope, will be found 
to be electrical. The linen faced with paper used in 
making strong envelopes and for paper collars, shows 
this very well. Lumps of sugar, crunched in the dark 
between the teeth, exhibit pale flashes of light. The 



78 ELECTEICITY AND MAGNETISM part i 

sudden cleavage of a sheet of mica also produces sparks, 
and both laminae are found to be electrified. 

69. Crystallization and Solidification. — Many sub- 
stances, after passing from the liquid to the solid state, 
exhibit electrical conditions. Sulphur fnsed in a glass 
dish and allowed to cool is violently- electrified, as may 
be seen by lifting out the crystalline mass with a glass 
rod. Chocolate also becomes electrical during solidifica- 
tion. When arsenic acid crystallizes out from its solu- 
tion in hydrochloric acid, the formation of each crystal 
is accompanied by a flash of light, doubtless due to an 
electrical discharge. A curious case occurs when the 
sulphate of copper and potassium is fused in a crucible. 
It solidifies without becoming electrical, but on cooling 
a little further the crystalline mass begins to fly to 
powder with an instant evolution of electricity. 

70. Combustion. — Yolta showed that combustion 
generated electricity. A piece of burning charcoal, or a 
burning pastille, such as is used for fumigation, placed 
in connexion with the knob of a gold-leaf electroscope, 
will cause the leaves to diverge. 

71. Evaporation. — The evaporation of liquids is 
often accompanied by electrification, the liquid and 
the vapour assuming opposite states, though apparently 
only when the surface is in agitation. A few drops 
of a solution of sulphate of copper thrown into a hot 
platinum crucible produce violent electrification as they 
evaporate. 

72. Atmospheric Electricity. — The atmosphere is 
found to be always electrified relatively to the earth : 
this is due, in part possibly, to evaporation going on 
over the oceans. The subject of atmospheric electricity 
is treated of separately in Lesson XXV. 

73. Pressure. — A large number of substances when 
compressed exhibit electrification on their surface. Thus 
cork becomes 4- w T hen pressed against amber, gutta- 
percha, and metals; while it takes a — charge when 



chap, i PYRO-ELECTRICITY 79 

pressed against spars and animal substances. Peclet 
found the degree of electrification produced by rubbing- 
two substances together to be independent of the pressure 
and of the size of the surfaces of contact, but depended 
upon the materials and on the velocity with which they 
moved over one another. Rolling contact and sliding 
friction produced equal effects. 

74. Pyro-electricity. — There are certain crystals 
which, while being heated or cooled, exhibit electrical 
charges at certain regions or poles. Crystals thus elec- 
trified by heating or cooling are said to be pyro-electric. 
Chief of these is the Tourmaline, whose power of attract- 
ing light bodies to its ends after being heated has been 
known for some centuries. It is alluded to by Theo- 
phrastus and Pliny under the name of Lapis Lyncurius. 
Tourmaline is a hard mineral, semi-transparent when 
cut into thin slices, and of a dark green or brown colour, 
but looking perfectly black and opaque in its natural 
condition, and possessing the power of polarizing light. 
It is usually found in slightly irregular three-sided 
prisms which, when perfect, are pointed at both ends. 
It belongs to the " hexagonal " system of crystals, but 
is only hemihedral, that is to say, has the alternate 
faces only developed. Its form is given in Fig. 47, where 
a general view is first shown, the two ends A and B 
being depicted in separate plans. These two ends differ 
slightly in shape. Each is made up of three sloping faces 
terminating in a point. But at A the edges between 
these faces run down to the corners of the prism, while 
in B the edges between the terminal faces run down to 
the middle points of the long faces of the prism. The 
end A is known as the analogous pole, and B as the 
antilogous pole. While the crystal is rising in tempera- 
ture A exhibits + electrification, B — ; but if, after hav- 
ing been heated, it is allowed to cool, the polarity is 
reversed ; for during the time that the temperature 
is falling B is -f and A is — . If the temperature is 



80 



ELECTRICITY AND MAGNETISM part i 



steady no such electrical effects are observed either at 
high or low temperatures ; and the phenomena cease if 
the crystal be warmed above 150° C. This is not, how- 
ever, due to the crystal becoming a conductor at that 
temperature; for its resistance at even higher tempera- 
tures is still so great as to make it practically a non- 
conductor. A heated crystal of tourmaline suspended 
by a silk fibre may be attracted and repelled by electri- 
fied bodies, or by a second heated tourmaline ; the two 
similar poles repelling one another, while the two poles 







-MB 



Fig. 47. 



Fig. 4S. 



of opposite form attract one another. If a crystal be 
broken up, each fragment is found to possess also an 
analogous and an antilogous pole. 

Many other crystals beside the tourmaline are more 
or less pyro-electric. Amongst these are silicate of zinc 
("electric calamine"), boracite, cane-sugar, quartz, tar- 
trate of potash, sulphate of quinine, and several others. 
Boracite crystallizes in the form shown in Fig. 48, which 
represents a cube having four alternate corners truncated. 
The corners not truncated behave as analogous poles, the 
truncated ones as antilogous. When a natural hexagonal 
prism of quartz is heated its six edges are found to be -f 
and — in alternate order. 



CHAP. I 



PIEZO-ELECTRICITY 



81 



75. Piezo-electricity. — In certain crystals pressure 
in a particular direction may produce electrification. 
Haiiy found that a crystal of calcspar pressed between the 
dry fingers, so as to compress it along the blunt edges of 
the crystal, became electrical, and that it retained its 
electricity for some days. He even proposed to employ a 
squeezed suspended crystal as an electroscope. A similar 
property is alleged of mica, 

topaz, and fluorspar. If two 
opposite edges of a hexagonal 
prism of quartz are pressed 
together, one becomes +, the 
other — . Pressure also pro- 
duces opposite kinds of electri- 
fication at opposite ends of a 
crystal of tourmaline, and of 
other crystals of the class 
already noticed as possessing 
the peculiarity of skew-sym- 
metry or hemihedry in their 
structure. Piezo-eleclricity is 
the name given to this branch 
of the science. It is known 
that skew-symmetry of struc- 
ture is dependent on molecular 
constitution ; and it is doubt- 
less the same peculiarity which 
determines the pyro-electric 
and piezo-electric properties, 
as well as the optical behaviour 
of these crystals in polarized 
light. 

76. Animal Electricity. — 
Several species of creatures 

inhabiting the water have the powder of producing 
electric discharges physiologically. The best known of 
these creatures are the Torpedo, the Gymnotus, and the 

G- 




Fig. 49. 



82 ELECTRICITY AND MAGNETISM part i 

Silurus. The Raia Torpedo,* or electric ray, of which 
there are three species inhabiting the Mediterranean and 
Atlantic, is provided with an electric organ on the back 
of its head,' as shown in Fig. 49. This organ consists of 
laminae composed of polygonal cells to the number of 800 
or 1000, or more, supplied with four large bundles of 
nerve fibres ; the under surface of the fish is — , the upper 
+ . In the Gymnotus electricus, or Surinam eel (Fig. 50), 
the electric organ goes the whole length of the body from 
tail to head. Humboldt gives a lively account of the 




Big. 50. 

combats between the electric eels and the wild horses, 
driven by the natives into the swamps inhabited by the 
Gymnotus. It is able to give a most terrible shock, and 
is a formidable antagonist when it has attained its full 
length of 5 or 6 feet. In the Silurus the current flows 
from head to tail. 

Nobili, Matteucci, and others, have shown that nerve- 
excitations and muscular contractions of human beings 
also give rise to feeble discharges of electricity. 

77. Electricity of Vegetables. — Buff thought he 
detected electrification produced by plant life ; the roots 
and juicy parts being negatively, and the leaves posi- 
tively, electrified. The subject has, however, been little 
investigated. 

* It is a curious point that the Arabian name for the torpedo, ra ad, 
signifies lightning. This is perhaps not so curious as that the Electra of 
the Homeric legends should possess certain qualities that would tend to 
suggest that she is a personification of the lightning. The resemblance 
between the names electra and electron (amber) cannot be accidental. 



chap, i ELECTRIFICATION BY CONTACT 



83 



78. Thermo-electricity. — Heat applied at the junc- 
tion of two dissimilar metals produces a flow of elec- 
tricity across the junction. This subject is discussed in 
Lesson XXXV. on Thermo-electric Currents. 

79. Contact of Dissimilar Metals. — Volt a showed 
that the contact of two dissimilar metals in air produced 
opposite kinds of 

electrification, one 
becoming positively, 
and the other neg- 
atively, electrified. 
This he proved in 
several ways, one of 
the most conclusive 
proofs being that 
afforded by his con- 
densing electroscope. 
This consisted of a 
gold-leaf electroscope 
combined with a 
small condenser. A 
metallic plate formed 
the top of the electro- 
scope, and on this 
was placed a second 
metallic plate fur- 
nished with a handle, and insulated from the lower one 
by being well varnished at the surface (Fig. 51). As the 
capacity of such a condenser is considerable, a very feeble 
source may supply a quantity of electricity to the con- 
denser without materially raising its potential, or causing 
the gold leaves to diverge. But if the upper plate be lifted, 
the capacity of the lower plate diminishes enormously, 
and the potential of its charge rises as shown by the 
divergence of the gold leaves.* To prove by the con- 

* Formerly, this action was accounted for by saying that the electricity 
which was " bound " when the plates of the condenser were close together, 




Fig. 51. 



84 ELECTRICITY AND MAGNETISM part i 

densing electroscope that contact of dissimilar metals does 
produce electrification, a small compound bar made of 
two dissimilar metals — say zinc and copper — soldered 
together, is held in the moist hand, and one end of it is 
touched against the lower plate, the upper plate being 
placed in contact with the ground or touched with the 
finger. When the two opposing charges have thus col- 
lected in the condenser the upper plate is removed, and 
the diverging of the gold leaves shows the presence of 
a free charge, which can afterwards be examined to see 
whether it be -1- or — . Instead of employing the copper- 
zinc bar, a single voltaic cell may be connected by copper 
wires to the two plates. For a long time the existence of 
this electrification by contact was denied, or rather it was 
declared to be due (when occurring in voltaic combina- 
tions such as are described in Lesson 
XIII.) to chemical actions going on; 
whereas the real truth is that the 
electricity of contact and the chemical 
action are both due to molecular con- 
ditions of the substances which come 
into coni act with one another, though 
we do not yet know the precise nature 
of the molecular conditions which give 
rise to these two effects. Later experiments, especially 
those made with the modern delicate electrometers 
of Lord Kelvin, put beyond doubt the reality of 
Volta's discovery. One simple experiment explains 
the method adopted. A thin strip or needle of metal 
is suspended so as to turn about a point C. It is 
electrified from a known source. Under it are placed 
(Fig. 52) two semicircular disks, or half -rings of dissimilar 



becomes "free" when the top plate is lifted up; the above is, however, a 
more scientific and more accurate way of saying the same thing. The 
student who is unable to reconcile these two ways of stating the matter 
should read again Articles 40 and 55, on pp. 46 and 6S. A much more sensi- 
tive apparatus to show the effect is the quadrant electrometer (Art. 288). 




chap, i CONTACT SERIES OF METALS 



85 



metals. Neither attracts or repels the electrified needle 
until the two are brought into contact, or connected by a 
third piece of metal, when the needle immediately turns, 
being attracted by the one that is oppositely electrified, and 
repelled by the one that is electrified similarly with itself. 
80. Contact Series of Metals (in Air). — Volt a 
found, moreover, that the differences of electric potential 
between the different pairs of metals were not all equal. 
Thus, while zinc and lead were respectively — and — to 
a slight degree, he found zinc and silver to be respec- 
tively — and — to a much greater degree. He was able 
to arrange the metals in a series such that each one 
enumerated became positively electrified when placed in 
contact in air with one below it in the series. Those 
in italics are added from observations made since Yolta's 
time — 

+ Sodium, Copper, 

Magnesium, Silver. 

Zinc, Gold, 

Lead, Platinum, 

Tin, — Graphite (Carbon). 

Iron, 

Though Volt a gave rough approximations, the actual 
numerical values of the differences of potential in air for 
different pairs of metals have only lately been measured 
by Ayrton and Perry, a few of whose results are tabu- 
lated here — 



Zinc 

Lead 

Tin 

Iron 

Copper 

Platinum 



Carbon 



Difference of Potential 
(volts). 

210 



069 



313 



146 



238 



113 



86 ELECTRICITY AND MAGNETISM part i 

The difference of potential between zinc and carbon 
is the same as that obtained by adding the successive 
differences, or 1-09 volts.* Volta's observations may 
therefore be stated in the following generalized form, 
known as Volta's Law. The difference of potential be- 
tween any two metals is equal to the sum of the differences 
of potentials between the intervening metals in the contact- 
series. 

It is most important to notice that the order of the 
metals in the contact-series in air is almost identical 
with that of the metals arranged according to their 
electro-chemical power, as calculated from their chemical 
equivalents and their heat of combination with oxygen 
(see Table, Art. 489). From this it would appear that 
the difference of potentials between a metal and the air 
that surrounds it measures the tendency of that metal 
to become oxidized by the air. If this is so, and if (as 
is the case) the air is a bad conductor while the metals 
are good conductors, it ought to follow that when two 
different metals touch they equalize their own potentials 
by conduction but leave the films of air that surround 
them at different potentials. All the exact experiments 
yet made have measured the difference of potentials not 
between the metals themselves, but between the air near 
one metal and that near another metal. It is certain 
that while in air iron is positive to copper, but in an 
atmosphere of sulphuretted hydrogen, iron is negative to 
copper. Mr. John Brown has lately demonstrated the 
existence on freshly-cleaned metal surfaces of films of 
liquid or condensed gases, and has shown that polished 
zinc and copper, when brought so near that their films 
touch, will act as a battery. 

81. Contact Actions. — A difference of potential is 
also produced by the contact of two dissimilar liquids with 
one another. 

* For the definition of the volt, or unit of difference of potential, see 
Art. 254. 



chap, i CONTACT ACTIONS 87 

A liquid and a metal in contact with one another also 
exhibit a difference of potential, and if the metal tends 
to dissolve into the liquid chemically there will be an 
electromotive force acting from the metal toward the 
liquid. 

The thermo-electric difference of potential at a junc- 
tion of two metals is a true contact difference. It is 
measured by the amount of heat produced (see Peltier- 
effect, Art. 420) by passing a current of electricity in the 
reverse direction through the junction. 

A hot metal placed in contact with a cold piece of 
the same metal also produces a difference of potential, 
electrical separation taking place across the surface of 
contact. 

Lastly, it has been shown by Professor J. J. Thomson 
that the surface of contact between two non-conducting 
substances, such as sealing-wax and glass, is the seat of a 
permanent difference of potentials. 

82. Magneto-electricity. — Electric currents flowing 
along in wires can be obtained from magnets by moving 
closed conducting circuits in their neighbourhood. This 
source is dealt with in Art. 222, Lesson XVIII. 

83. Summary. — We have seen in the preceding 
paragraphs how almost all conceivable agencies may pro- 
duce electrification in bodies. The most important of 
these are friction, heat, chemical action, magnetism, and 
the contact of dissimilar substances. We noted that the 
production of electricity by friction depended largely 
upon the molecular condition of the surfaces. We may 
here add that the difference of potentials produced by 
contact of dissimilar substances also varies with the 
temperature and with the nature of the medium (air, 
vacuum, etc.) in which the experiments are made. 
Doubtless this source also depends upon the molecular 
conditions of dissimilar substances being different ; the 
particles at the surfaces being of different sizes and 
shapes, and vibrating with different velocities and with 



88 ELECTRICITY AND MAGNETISM part i 

different forces. There are (see Art. 10) good reasons 
for thinking that the electricity of friction is really due 
to electricity of contact, excited at successive portions of 
the surfaces as they are moved over one another. But of 
the molecular conditions of bodies which determine the 
production of electrification where they come into contact, 
little or nothing is yet known. 



K931 










V 












1 V- 






c^ 


















vV 















.* N 












- ^ 






O X 


















^M 




